it means if she grows fat..she wants you to be fatter than her
so she wun feel fat..and if she grows skinny, she wants you to
still remain fat so at least she knows she is skinnier than
you..and other guys would not think that you are her bf and that
other guys still have a chance to date her..
in other words, she wants you to be her meat shield, her lamp
post, her convenient excuse..
sounds like a wing man to me 
