I would have to agree with barb on this one. Different foods have different amounts of calories. I had Milenko (4 mnths old) on Nutro and he had to eat 3 cups a day and he would poop like crazy.
I recently switched him to Canidae. It was reccomended to me by my pet store for pit bulls because it helps build strong muscles and it doesnt contain any by-products or any fillers. The food also has alot more calories per serving (i think about 470 cal/serving) so I now only have to feed him 1 1/2 cups a day. Therefore he is pooping less now which is alot better and it actually makes the poop smell less too
Canidae costs a little bit more ( I think I just paid $22 for 15 or 20 lbs) but it lasts longer because you feed them less and the few extra dollars is worth it because the quality of the food is so much better than stuff like IAMS and Pedigree.