All time great heavyweight champions best fighting weights...

you can hit a lighter object much further than you can hit a heavier object. If you are heavyier than the object you are hitting, you will be able to transfer your weight better and get a decent shot on the object. So in this sence, it is believable that pure fat can make it much harder for a smaller opponent to hit you as well.

Not sure if that explanation has any merit, but it seems a possibility, sort of.