An equal number of people on each side does not make teams even. I played one map against only 8 people, who were excellent players, vastly outnumbered, and they still beat us... An even map is down to the skills of the players on each side.
I often change sides to a team with less players.... and I find many players willing to do the same. I think in general the players will be willing to move about If a team is too easy to defeat, just so the balance can be right.