actually some of the groups are that large, but a lot of the track ones are a like a dozen or so depending on the agent. look at colm's group, templeton, not so big.
another thing is that in the super large groups there isn't so much concern about everyone improving, they can afford to waste some athletes through injury or burnout because you're guaranteed to succeed with a select few. that works for kenya because there are so many potential elites sitting around. in america we don't have that luxury obviously.