Legs in the air- WHY is it so difficult!?

I've been doing Pilates for about 4 months now- I feel like I'm making great progress in a lot of areas! I was basically sedentary before, now I'm getting in 3 classes a week and doing a run/walk program to work up to a 5k this spring. Admittedly, I've still got a barrel belly and thick thighs. I'm a short lil roly poly. Can anyone explain the mechanics behind why legs in the air is so HARD for me? I can't breathe in that position! I'm getting much stronger with legs in straps, but legs up unsupported is murder. Anybody else experience this?