Knowing my rights as a Marine being hared with BCP?

You're a new comer, it's something that we all go through, but for them to talk about your weight and call you fat is wrong. I'd suggest you fix the problem at the lowest level possible and if it doesn't get fixed, work your way up the chain of command. As long as you know that you are within regs, you have NOTHING to worry about and they can't touch you, but make sure that you come correct when you mention your situation. Although your superiors are in the wrong, remember, they have been in longer than you. Talk to your supervisor first, if that doesn't help, go to your supervisor's supervisor and so forth, even if you have to go from enlisted to officer. I'm sure if they know that you mean business, it will stop.