Have to understand that fatties have to compensate somehow to be accepted or validated the same as more attractive women in life, so telling everyone she's a boyfriend makes her 1) seem like someone who isn't that disgusting that she repells ALL men, if some man apparently wants her she probably isn't a complete headcase and 2) Makes her insanely proud so she can't help herself and 3) Fat girls talk ALOT anyway so why would they shut their mouths at the opportunity to brag?
Of course it all comes from insecurity.
Getting acceptance from the 'herd', that's what it all boils down to.