Sleeping bag warmth vs. fit

Sometimes I like to pull my legs up into fetal position – sleeping bag has to be a little bigger – obviously this makes me warmer

I don't think anyone will have the in the field experience you are asking for. After all, you'd have to own 2 identical bags except for size and use them several times in identical conditions. Who has those bags and who has that experience?

Maybe a bag maker? Maybe army research?

Lacking this knowledge, I would suggest 3 strategies for picking a bag size:

1 get the slimmest you can comfortably sleep in, most warmth for least weight

or

2 get an extra wide one, if it's warm the extra space will be nice, if it's cold the extra space will allow you to wear clothes inside

or

3 get a quilt/variable girth bag/stretch bag (like Montbell).

"I don't think anyone will have the in the field experience you are asking for. After all, you'd have to own 2 identical bags except for size and use them several times in identical conditions. Who has those bags and who has that experience?"

Thanks for asking the clarifying question. What I was looking for was something like: "I purchased this slim bag and now I really regret it because …" ; or, "I purchased a wide bag and I'm really happy I did because …";

I'm a slim 166 lbs and at 6'1" with size 13 feet I've always used a "Long" 6'6" bag. Have I tried squeezing into the 6" bags ? Yes – it's just not comfy for my sleeping style — as I'm prone to tossing and turning throughout the night.

I did the A.T last year in a Western Mountaineering 35* Caribou Long (65"/56"/39") (inside girth of Shoulder/Hip/Foot) and was comfortable in that I could move around fairly well in the bag without feeling like a was restricted. I have since switched to the 30* Megalite with the exact same specifications for inside girth, and am happy with it as my go-to 3 season bag. And for the cooler fall temps, my bag is the 20* Alpinlite – that also has the exact same specifications for inside girth.

Do I think I lose a little warmth with a roomy bag ? YES – without a doubt. I'd guess around 5* is lost of comfort rating due to the roominess of the bag. Well worth it when I stick all my extra clothing and electronics inside the bag at night to keep them warm.

Even roomier for my Winter bag ? Yes – WM 0* Kodiak Long has the inside girth measurements of (67"/58"/41") which allows for a baselayer to be worn also (and with a VBL can get this down to -20 without a problem).

for me, the warmer the bag, the bigger i want it, but still in a mummy cut. i find that in cold weather conditions (<10*f) i really like a bag that i can get dressed in and one that i can keep things like a hot water bottle (or two), inner-boots, gloves, and pants in. from experince, i know that i'm much less likily to be changeing clothes in a 20* bag and that i'm ok with a slimmer cut. if i'm pushing the lower limit in the 20* bag i can still get in it with all my clothing on and drape my insulation layer over the top of the bag. this is what i like and what works for me. you need to figure out what works for you.

I just went through this exact decision process this past summer. I wanted a nice 20* bag and narrowed my choices to FF and WM. Then had to decide between the FF Hummingbird, FF Swallow, WM Ultralite, and WM Alpinlite. I finally decided on FF in the end, but again, which one. I ordered the Hummingbird.

When I got it I was shocked at how snug it was. I was worried so I ordered the Swallow to replace the Hummingbird. It came in and I spent a weekend trying them both on over and over (in the clean comfort of my house, of course) and finally decided that while the Swallow was bigger it wasn't necessarily more comfortable. I was still in a mummy bag that was smaller than most all bags from mainstream companies like Marmot and Mountain Hardware. I liked the custom, glove-like fit of the Hummingbird so I kept it and sent the Swallow back.

I've now used the Hummingbird twice in (relatively) warm mid-30's conditions. All I can say is it's the warmest bag I've ever slept in. Maybe a little too warm. I don't usually have to zip it up all the way. One time I did and woke up around 2am bathed in sweat. I had to get out of the bag and change to a set of dry baselayers to get back to sleep.

Of course, some of it is due to the high-quality of the bag as a whole, but some has to be the space efficiency. As soon as the zipper is secured all the way the bag is very warm.

I have a WM megalite, its very wide. Even more on me after dropping ~40 lbs, than it was when I got it.

I can feel the cold at the sides of me when pushing the limits of the bag. However, when its cold I keep my clothing in the bag, on the sides of me. It takes up the room there, blocks that cold, gives me something to rest my elbows on to be more comfortable too if lying on back. Then If I want to put it on, its easy to do, better than getting a long bag and stuffing clothes at your feet.

The drawback, its heavier, and bulkier than it has to be.

I can also fit my 40F quilt inside the bag with me. The combined loft is crazy, like 6", a giant puffy cocoon. The combo is heavier than a bag good for single digits-teens, but is versatile.

I have a roomy synthetic mummy. When it's cold, I tuck the extra width under my torso and between my knees. It seems about 10-15 degrees warmer when snug.

I also have a roomy down mummy. It's warmer since it drapes better. Tucking it tight around me adds 5-10 degrees of warmth though.

My opinion is that in the bag room vs warmth trade-off, you loose the most warmth in a cold (30-40F) light bag. In my experience (MH Phantom 32), such a bag tends to float above you instead of hugging you. A warm heavy bag (REI Downtime 20) collapses and hugs you minimizing dead space.
A wide comfortable bag is not so comfortable anymore when you're cold.

snugger bags are warmer all other things being equal …

If you regularly toss and turn, get a looser fitting bag and you'll stay warmer. If you are claustrophobic, avoid mummy bags or other bags that shroud your head. A small bit of dead air space actually helps keep you warm. And remember when it's really cold you'll be wearing (or should be wearing) extra clothes. Beware the tight fitters for 10 F and below.

those with engineering backgrounds will understand that surface area is proportional to the heat transfer, ie, with same insulation thickness, and same temp differential, you lose more heat when one bag has more surface area than another. Larger bags have more surface area, lose more heat. It takes a larger heat source inside to make up for that.

I personally use a sleeping bag that fits me, not much room to move around inside the bag.
This works fine for me and I flip flop all night. I don't move within the bag, the bag moves with me. So basically I wear the bag.

I can curl up in a fetal position or whatever as long as I expect the bag to also go into the fetal position.

This is a much warmer scenario than using a bag so big that you slide around inside.

The only problem is that I can't spread my legs or arms, but that really isn't an issue.