There are two sizes of tube trains - "deep tube" and "sub surface" trains. The deep tube trains are much smaller, and they are difficult to stand in if you're near the doors for anyone who's over 6 feet (I'm 6'4"). That said, it's not the end of the world, as you're only standing there when it's crowded, but it does happen.
See here: http://en.wikipedia.org/wiki/London_Underground#Trains