I'm a big lad myself - no amount of weight loss will alter that; I'm just well-built. It does rule out the 152 which does seem to be designed for midgets. I'm sure most 4 seaters will be fine. After all, most of them are American planes and the US is the lard capital of the world.
Being overweight is so hard to avoid these days. Lack of time, sedentary occupations, and the availability of convenience food see to that. Sure, you can use flying as an incentive to lose a bit of weight, but if you don't want to then **** what everybody else says. I've lost a bit of weight over the last few months and it's a right pain in the ass - you have to go shopping for new clothes...