I'm not a biologist, but I do take an active interest in the study of fish behavior - particularly of trout. Through the years I've observed, as I'm sure you have, trout acting in a way that didn't make sense. Wanting to know what would cause trout behavior strange to me I try to find the answers through research. When that doesn't work I ask an expert. Talking with a fish biologist usually leads to more questions from me, but I always learn something new & interesting (I suppose it will get to the point where the PA Fish Commission folks will cringe when they see me coming, if it hasn't already).
A while back I had read, probably in a book by Edward R Hewitt, that given a choice of food a trout of average size will always select a nymph over a smaller fish. The thinking was that nymphs have a higher percentage of fat, thus more calories, in relation to their size. I'm kinda skeptical of this for a few reasons & I suspect this may be true only in a perfect world for the trout.
I've read, & been told by fish biologists, that trout are opportunistic eaters (a lot like me). If they can get a meal with very little effort on their part & with little risk, they will (again, a lot like me). In other words, a trout will almost always go for the biggest meal for the least amount of work. Could this be one reason why they sometimes get very selective when one type of food or bug is in abundance?
I've also had it explained to me that trout will gorge themselves on all foods when it's readily available. The idea of a trout being full or satiated with food is possible, but rare if conditions are favorable for them to feed.
This was proven to me one time when I decided to keep a couple of trout for my supper - I practice catch & release but I'm not against keeping a few for the frying pan once in a blue moon. Anyway, upon cutting one of my trout open I noticed that it's stomach was misshapen. Thinking something might be wrong with the fish, I cut the stomach open to see what was the matter. It turned out that nothing was wrong, the stomach was just jammed full. I found a lot of nymphs, some vegetation, & a dace (bait fish). I couldn't imagine this trout had anymore room in its stomach for anything at all, & yet it took my fly with the intention of packing it into it's belly with everything else it had in there!!
So, how much can a trout eat? A lot I guess. I also suppose that's good news for us anglers, too. Although it probably ruins my excuse for not catching any trout when I say "they must be full from eating yesterday's hatch"......maybe I'll just have to start blaming the weather!!!