compressing marshmallows?

They will never be the same again – you can do it…but YMMV.

Ha, ha. You can't. They will simply expand to replace whatever air you suck out.
You could try dehydrating them….

You can totally compress marshmallows*, and you're very close to the right idea with the vacuum bag.

A couple months ago, I bought a vacuum pump and bell to degas liquid silicone (to prototype the custom stopper for my Backcountry Boiler :) ). Remembering middle school science class, I tested the vacuum I was drawing by putting marshmallows in the bell. At first (as James points out), they get huge as the air bubbles expand. But then the air bubbles burst, and when you remove the vacuum, they're about half their usual size and really dense. As Sarah points out, they'll never be the same, but it's a lot of fun!

*I'm quite sure this is not helpful at all, but I wanted to share. :)

Yeah, Devins correct, you could try it two or three times and see what happens.

Another problem, if it's a warm day, is that squished (or compressed, if you prefer) marshmallows tend to fuse together. You may end up with one enormous marshmallow!

Looks like my bag is going to be taken up by more than 1/2 by marshmallows. On the plus side besides doing something for the CCC my pack will be 2x as light when I drop them off. Thanks everyone.

…Aren't the marshmallows multi-use? Sleeping pad, pillow, place under jacket for extra insulation, etc?

I tried this in a Food Saver bag a few months ago to take on a trip and the marshmallows ended up being one big gooey lump. I could pry them apart sort of, but it was mostly fail. On top of that the package didn't really compress at all. Good luck.