Apart from what kansas said, have you ever thought of breaking them in? Im assuming if you have a gelding that he would be old enough to be broken but I can't comment on the mare. Is she old enough to be broken in??
Apart from chasing them around the paddock, which obviously isnt a suitable idea, breaking them in and working them could be the only thing I can think of. Just like people, if we don't exercise yet keep eating we will get fat and the only thing to counteract that would be exercise.
Aside from a health issue, lack of exercise would be the only thing I would consider as being a cure for this. It doesnt sound like they get overfed. Maybe you could cut out the hard feed and just give them their roughage
as we know, horses only need roughage. Their hard feed is only something we give them to help them along a bit :) if they are already overweight I would consider dropping the hard feed.
This is what I would do after ensuring that there are no health issues at hand but considering there are two horses showing this fatness I would lead more towards not enough exercise :)