Perhaps. It seems that the other parasitic-types are still adult men to tend to eat a lot anyway, so maybe it just wouldn't have been noticed. (Your explanation of them vs. Allen made me laugh. xD) I wonder if he did have infrequent meals, and that's why he's... well... not really on the tall side?
Now for the amount people eat... yah, Allen really does enjoy it (food in general) but I think that (and maybe why he eats so much) is mostly psychological. I mean he had to have infrequent meals doing the travaling clown ruteine, so maybe it stems from that. However, he (and his innocence) is obviously still growing, so there is that. Lenalee is twards the end of the growing cycle, so she probably won't eat as much, but if her innocence uses blood, then she might eat/drink almost as much to replenish that. As far as the other parasyte types, I wasn't trying to say that they ate as much as Allen (far be it) but they might regularly have thirds or fourths, instead of Allen's "I'll have everything on the menu... twice!" or others first/second trips.