That sounds interesting. I don’t think it was ever mentioned, but a species that can evolve to consume and metabolize everything, evidently utilizes all the food for energy without any waste. Therefore, no need to go to the bathroom. Matter-Eater lad and other Bismollians have a super metabolism, that accelerates the more matter is consumed. Therefore, they don’t get fat. Although MEL once caught part of a nuclear ray blast aimed at Superboy and his metabolism slowed so that he resembled Bouncing Boy (another hero with powers of dubious usefulness). A kindly professor whipped up a metabolic enhancing formula for him and he regained his svelte appearance.
Katie “Energizer” Powers, from Marvel’s Power Pack, has essentially the same ability. She “ingests” by converting matter into energy and then absorbing the energy (which, come to think of it, so do you and I in a less-direct manner). I suspect Matter-Eater Lad does pretty much the same thing, no super-jaws or -intestines required.
As a matter of fact, it was.
I have this unfortunate habit of mulling over intentionally lame or “comedic” character concepts like Matter-Eater Lad and trying to figure out if they could somehow be made cool. What if, instead of literally eating stuff, he could generate a field around himself similar to a black hole’s event horizon, so that any physical object passing too close to him would be drawn in and annihilated? Bang, he goes from comic relief to hellaciously dangerous opponent: “Grife! Fire all retros, full power! The ship’s being pulled in by… the Matter-Eater!”
I envision something along the same lines for Bouncing Boy-- instead of the patently goofy inflating ability, he might be able to surround himself with a protective force field, enabling him to reflect, augment, or redirect kinetic energy (effectively the same power, only it would look cooler). Alternately, perhaps he could transform himself into a highly resilient sphere of living force.
I’m still working on Color Kid.
Terrifel:
He was used to great effect in the TMK issues, particularly # 26.
I know, I know, hijack, I just wanted to say that brother…I do the same thing. You are not alone. 
And Color powers can be hella dangerous, at least in our century. So in the future, probably just as much, if you’re creative.
But to consume several times his own mass without bulging in the middle like a python that just swallowed an elephant, he’d have to get rid of that mass/energy really fast somehow. Most likely by radiating it. So if he’s eating, his skin should be burning hot. If he can live through that, that’s another special ability of his species – they must be able to swim in molten lava without injury.