I have a three down jackets that I use for backpacking, so can compare and contrast a bit. All three have a similar lightweight shell, but the amount of down varies considerably. The warmth of the jacket is all about the amount of down.
My warmest jacket is a Mountain Hardwear Phantom. I bring it in the winter or cold shoulder season. Yes, I look like the Michelin Man in this jacket, but who cares, wearing this jacket I am NEVER cold. Weighs 14 ounces.
My middling warm jacket is a Outdoor Research Women's Aria jacket. It is similar to my friends Marmot Venus jacket and weighs 11 ounces. This is not nearly as warm as the Phantom but generally is quite sufficient for the Sierras in the summer.
I also have the Eddie Bauer First Ascent Men's jacket in the same nice blue color as Eric. The weight it's the same as the Outdoor Research or the Marmot Venus.
The advantage of a man's jacket is that it has a lot longer back length so better coverage, more warmth. Not as flattering obviously.
Finally my friend brought along her brand new Patagonia Ultralight Down Jacket on our last trip. This weighs an amazing 7 ounces and felt almost weightless. However she felt cold very soon after the sun went down and had to crawl into her sleeping bag even though it was quite a pleasant May evening.
Three takeaways:
If you really want a very warm jacket, get something puffy that weighs close to a pound.
For three season use you'll be fine with something in the 10-12 ounce range.
The lightest jackets are amazing, but really don't add much warmth.