The are plenty of guys in the NFL that are not too big for soccer. If they were playing soccer, they would be leaner because they wouldn't need the muscle mass required for football. Their workout regimens would change to meet their sport.
Talent is an issue, but not the main one. And I am talking pure physical athleticism(speed, quickness, agility, explosiveness and some size). You don't choose players only because they are great athletes, but being a great athlete never hurts.
The most important issue is skill development. Develop that aspect and you can overcome not having a bunch of top end athlete. I agree that the college route is a waste, but it starts even before they get to that level. Academies can be a huge key. Get them young and working on their skills. I also think it would be nice to increase the player pool. If you can get more kids playing the game at all ages, then you increase the talent pool. You can find the guys that are more adept at the skills, the guys that are the better athletes and most importantly the guys that are both when you get more and more kids involved in the game and attending the academies.
U.S. soccer has plenty of good enough athletes. They are not slouches. It would be nice to have a couple of truly great athletes out there. I would not trade off a very skilled player for a great athlete, but I would like to see more guys who are both.
I look at DeMarcus Ware. If you could find a guy with that kind of athleticism, but also had very high soccer skills, you could find a place for him on any soccer squad. A person like that wouldn't be as heavy as he is playing in the NFL because he wouldn't need to be, but he would still have good size and strength. That kind of guy could be a heck of a center back.
Finding the most skilled players and developing those skills to an even higher level is the main objective, but I am not turning away great athletes because they are too big. If they have the skills, they have the skills.