Guys friend zone a girl for the same reason girls friend zone a guy: because there is some attraction aspect that may be missing and the person, unfortunately, thinks they're being nice by not telling the other person the truth while still allowing the person they're friend zoning to hang onto a small margin of hope that something will work out for them to be together when there actually, probably isn't. Ok, maybe that's only my experience, but that tends to be the case. There is something, however, that you do have that the other person can't give up: like a bangin' personality.
Look, don't start getting down on yourself about your weight, because trust me for every guy that will look past you for that reason, there's someone who'll look past that for you. Trust and believe someone out there will be looking for you, just don't hang yourself up on one person (been there, made that mistake).
Now to answer your question:
1. The friend zone isn't some mystical prison where rejected people go to (romantically) die.
I personally wouldn't friend zone a girl, I would just tell her I'm not interested but still be friendly to her when we were around each other, but respect and not cross a boundary that would give her the wrong impression. (like I said I've been friend zoned and it sucks ass)
2. To get out is a very tricky thing. To be honest, if a person puts you in the friend zone you shouldn't want to try to be with that person. Never change who you are for someone else, because then they won't be falling for you they'll be falling for something that you're pretending to be.
3. Trust me, that part sucks. In my personal experience, I was friend zoned by two girls (on two separate occasions) who liked that I treated them like I would a girlfriend (because I really liked them): ie, I would take them to lunch/dinner, listen to them about their day, especially the bad days and try to fix it/ make them feel better, etc. However, they apparently (although both denied it) obviously thought (key word, 'thought') that I lacked something they were looking for. They figured they could keep me around in the friend zone and date guys that couldn't or wouldn't do those things because I would do the things their bfs wouldn't (they could have their cake and eat it too). I fell for one of these girls pretty hard (to be honest still not really over her) so it is pretty hard to see/think about her, but its kind of like getting shots: It's gonna suck, but eventually the pain will go away.