The Peace Corps has pretty specific BMI ranges for medical clearance before services. Let me know your height and I'll see what the maximum borderline weight you can be at to pass that portion of the medical clearance process.
5' 8"
Yeah, you need lose about 100 lbs to join the service.
Pretty much, yeah. The maximum weight for a 5' 8'' male is 185 lbs. It's not completely unheard of for someone who is borderline (like, say 186-195 lbs) to gain medical clearance, but anything above the max will require an additional battery of medical tests to determine if an underlying medical condition is present.
Regardless, I'm pretty certain that being 200+ lbs will lead to an automatic deferral.
Obviously, BMI isn't the only criteria used to determine medical clearance. I don't know anything about your medical history beyond what you and others have posted in this thread, but I'd highly advise looking at the conditions that the Peace Corps usually cannot accommodate before you pursue this venture any further.