self-imposed diet?

I have a silly food question --

I feed Frodo and Sam twice a day on a very regular schedule with Iams, and they have had no problems. They always scarf it all down. However, lately, for the last week, Frodo hasn't been finishing all his food. He's the big eater and that gives me pause. Otherwise he seems healthy enough, no litter box problems, etc. Can anyone think of why this would be? Unless his brother's been teasing him a little for being a bigger boy and he's trying to diet.

Perhaps he wants a little bit of variety? Jamie will eat a certain brand for about 3 months (though not every day - I have to give him three different brands of dry every week, and 3 or 4 different brands of canned, in all different flavors), and then he eats less and less of it, before refusing it entirely. I have to rotate brands with him. It's gotten to the point that I mark the "grand opening" of a new bag of dry food on the calendar. After 3 months, I "semi-retire" that brand, and replace it with another he'll eat. The only food he never seems to tire of is Eukanuba Lamb & Liver, which is okay, but not my preferred brand.

It is hard to say there are a number of reasons for a decrease in appetite. I'd try what Jcat suggested, and I'd be watching him like a hawk for any vomitting, increased urination, increased thirst, and change in stools - if I saw any of those or if you feel he's still declining in appetite/losing weight after trying a different food, get a vet exam done.

I think I will try out another brand of food for them. It seems now that after seeing Frodo leave food in his bowl, Sam is doing it too! I wonder if I can fool them with just another flavor of Iams, not drastically switching to a completely different brand.

That might work, or you could try Eukanuba, which is also made by Iams. If you switch foods, you should do it gradually, by mixing them, just in case they have sensitive stomachs.