Weight is just one facet of a good old fashion problem.
What's to insult really? Gender? Sexual preference? Weight? Height? Intelligence? Cognitive function? None of these things are really worthy of attack, but (at least since my childhood) they are all ripe for the picking.
I mean really... calling a person stupid nowadays? Who here doesn't know and love a person with low IQ who is a perfectly valid and worthy as human being?
IMO, the only permissible targets should be behavior and attitude. The only insult should be "asshole", because that's something that is actually bad and within a person's power to mediate. I mean, is there really anything else?