In my experience as a member of my troop and from what my friends tell me of their troops, the smaller the troop the easier it is to encourage Service Leadership. In our troop the Eagle Scouts usually stay until they go to college/age out. In some cases if they decide to go to tech school they will still come camping and teach scout skills. I'm from a smaller troop so everyone knows each other well, and even among us boys those who are in scouts only for the 'Eagle' designation are looked down upon. My friends in larger troops tell me that most of their fellow scouts leave soon after getting Eagle. I think at some level it is because in smaller troops older boys can feel as if they are making a difference and it is easier to connect to a troop as a whole. I've never been in a large troop (my troop has had a max of 7 people until last year) but from what I've been told smaller cliches form, and boys tend to hangout with a core group of friends. This lack of social mobility may make returning to the troop less desirable.
Another thing I've noticed is that there are only seem to be two types of boys who get Eagle Scout, those who have been in it for the Eagle from the beginning and are highly motivated for it, and those who love scouting, and gradually gravitate toward eagle, eventually working to complete it. The first usually earn it earlier, leave the troop, and don't truly enjoy scouting because they are always looking for the next rank, the second tend to enjoy the experience, truly benefit from it, and generally are the ones who make scouting a part of their identity, and strive to live by scouting values. Obviously their are exceptions but in my experience this has been the case.
^all of this may be purely anecdotal, but it is based on my observations.