There are many reasons why someone is overweight. Weight gain isn't just caused by eating; it is also caused by stress, lack of sleep (work/school can eat our hours of sleep), side effect from medications, genetics, etc. As an employed college student, I fall under the category of gaining weight from stress. Ever since graduating from high school (2010), I must have gained about fifteen pounds (150 to about 165); I have almost no time to exercise, because I work until midnight during weekdays at my job, and I need to make time to study. Where is the time to exercise in this schedule? NOWHERE. Thankfully, I'm done with school for the summer so I can have time to exercise, but that's beside the point. My point here is this: there are many reasons why someone is the way he/she is, number one reason being that he/she is born that way. For Pete's sakes, this is COSPLAY. COSTUME PLAY. You're not making a movie rip-off of the anime/game/whatever, so why the hell should you be born in a SPECIFIC way to DRESS UP? We cosplay to have fun, not to be "oh so ttly awsum i look like the character evry1 <3s me and my cosplay fck ya".
If you're black who wants to dress up as an albino, by all means go for it. If you're overweight who wants to dress up an anorexic character, no one is stopping you. You want to get in character? Buy/make the costume, put it on, and go for it.