It could be that the smaller fish know that if they stay out in the "open" too long, they will become prey to larger fish or from up above, so, "grab it" and run back to cover. The larger fish can move in on the bait and knows all the smaller fish will make room and it can just take it's time when feeding. Also, the younger fish have a lot more energy than the larger fish. Lets face it, we all moved faster in grade school than we did in high school!
Warren
Fly fishing and fly tying are two things that I do, and when I am doing them, they are the only 2 things I think about. They clear my mind.