Can Ants Get Fat?

Given that ants have an exo-skeleton, and all soft tissues are inside of that, if they do store body fat (which I doubt) then the more fat they have the less room they will have for body organs to function.  The exo-skeleton is one of the main factors limiting the growth of insects because when they are doubled in size, the amount of interior space being used for their breathing capillaries must increase by more than double.

They could be suffering from anorexia nervosa or bulimia nervosa.  I think you are right to be concerned.  My advice would be to watch them closely and see if they try to make themselves sick after they have eaten.  If you can, perhaps you should also try and weigh them every few days to see if they are still losing weight.  It maybe that you need to call in a specialist who has more experience of this sort of problem.

I know your not being serious...but I'll answer it anyway, just to make me feel smart. Ants have exoskeletons, some ants shed and grow new exo skeletons as they grow, but since their diet mainly consists of carbohydrates they have no need to store much fat, and even if they do their exo skeleton prevents them from being 'fat' so their size is generally the same.
 
If they are looking pale though, that might be a sign that they aren't getting enough to eat. Can't hurt to put more food out anyway.

I think that is normal for an ant. You probably can offer them food, but other than that I think that's how they are supposed to be.