Think of the roll up. If you have short legs and a long torso, you have a lot of weight to lift and not much to help counter balance you. No matter how strong you are, this exercise will always be harder for you that someone with longer legs or someone who is similar in size on the top and the bottom.
When I went to the psoas workshop last week I learned that depending on where the psoas connects to the spine it get affect how you move. The psoas can connect to your vertebra at T12 or L5 (which I knew), but what I didn’t know is that if it is lower (on L5) it will be easier for you to bend backwards in your spine. I’m really bending. When I was young I used to be able to touch my feet to my head (bending backwards). I can’t get my toes to my head anymore, but I can still bend backwards pretty far. There are a lot of exercises that we bend backwards in during a pilates class (swan dive, rocking, etc.). Where your muscles attach (something you don’t typically know) can make an exercise harder or easier.
The size and shape of your bones can make a difference, too. Depending on the length and angle of the femoral neck (the top of your thigh bone) can determine to some degree how far you can move your leg away from the body without moving your pelvis. So when we lie on our side and do leg lifts in class, some people are naturally going to get more range of motion—a range your body may not be able to shoot for and keep good form at the same time.
So next time your find yourself comparing yourself to someone else, cut yourself some slack. You are different; your body is different, and you deserve to be treated like you have a different body that is completely unique to you.