I don't see much reason to make this change.
I don't really see any reason why they should get less exp than a hero for the same work either, but I definitely don't see why a 3 character unit should get any less than a 5 character unit.
What is the reason that a 6 character unit should level half as fast as a 3 character unit? Does being around other people doing what you do make you collectively stupider?
In a classroom sense, kids learn more if they get more one on one time with the instructor, but on a battlefield it is one on one time 24/7. Everyone is in a learning environment every second of the battle. It isn't like person A shooting twice would prevent person B from also shooting twice in the same period of time. Person A, B, C, D, E, and F could all shoot twice in the same 2 second time period or whatever it is. If there were 100 people they could all shoot twice in the same 2 seconds or whatever.
To me this means that they should all learn at the same rate regardless of how many there are. It is that way in the game right now and it makes sense to me that it should be that way.
The only reason I can think to do it your way is to punish having better armies which seems to go completely against the whole concept of the game.