Hi A.,
This is indeed an unusual problem, but if your sons are starting to chunk up, it might indeed not be a good sign.
Babies and small children do indeed know when to stop, unless one or two things happen: They are fed with a bottle, and forced to finish even when they don't want to, so they unlearn the satiety (full) signals. Or there is something biological that interferes with that signal. In your place, I'd listen to my instinct that something is not right, and try to figure out if they were overfed as babies, were always eating and eating, if it started suddenly at one point (when they started to eat solid foods, for instance) - play the detective to find out when this started, if it was at the same time for both kids, etc.
I wouldn't limit how much they eat at one meal, but I would say no to food less than 2-3 hours after a meal, and see what happens.
Hope this helps,
K.