John, I can explain my experiences with three MBs, re: your question:
"If you are really small and hardly stretch it at all, wouldnt it be warmer than if you are larger and fully expand it?
- Yes, but they accomodate a 50th percentile male without much stretch (in my estimation). If you are overweight it might be a problem, but then you probably would not pay a premium for a montbell bag to save a few ounces.
"It seems like the loft would flatten out as it covers a larger area.
- Yes, certainly. Most the stretching and repositioning I do is temporary, or stretches the bag locally, for example where my knee is bent and jutting out. Overall though, the bag stays lofted, only stretching where necessary.
"So are Montbell's temp estimates for a bag that is fully expanded?
- I dont know, but general consensus is the temperature ratings are conservative, and correct for most males. My #7 is all I need down to 40'F with an insulation layer.
"I am interested in these bags because it seems like they would be easy to put inside another bag for really cold weather, but I want to hear what happens when they are fully expanded.
- Or, better yet, put it outside your other bag, since the MB will stretch to accomodate the inner bag. Putting a MB bag inside a non-stretching bag would render the super stretch system ineffective. Two MB bags, one inside the other would maintain the system also.
If you are concered about room or loss of loft, buy the long. I'm about 5 10 and went with the long on two of my three MBs, they stretch to 75" in the shoulders.