The fact that aliens win more in larger games is somewhat irrelevant, as there are MANY other things that depend on player count affecting the outcome. It doesn't take lots of humans to stop aliens from evolving, it happens with 6 vs 6 easily enough. Also larger teams = less chance of all large aliens getting killed off.
I'm not talking about situations where humans have an outpost at all alien base exits and have actually managed to get most of their team on the offensive (which would be the proper definition of 'dominating', and would actually win the game almost always if map control mattered), and that wouldn't even necessarily prevent aliens from evolving unless their base is in a tiny room.
I'm talking about situations where aliens have lost a few defences and humans have managed to get a couple of suits inside their base and kill off all/most large aliens (this is NOT "clearly dominating"), at which point no more large aliens appear, and aliens have a VERY difficult time getting the humans out again, even if they have plenty of evos. You can't simply go out of the base and come back at this point, by the time you get outside you will often be low on health making evolving a waste. Even if you manage to get out and back in at full hp, the few extra seconds it took you could have been used to kill off the few attacking humans if there were still several large aliens.
I'm not complaining about this because it makes aliens lose, but because it can take aliens from almost winning to about to lose too quickly and with little effort from the human team. Even if aliens can stop the humans from finishing off their base for a long time, they can't recover (without hugely outperforming the human team) since at this point all competent humans are constantly fed far more credits than they lose. Even if aliens manage to finally push humans out, they will already be at a huge disadvantage, with humans fed and on the way back, and alien base in shreds with most aliens still low on evos, since humans often focus on the few that did manage to evolve, who rarely stand a chance against several suits with just dretches backing them up.
Having matches well balanced by win ratio doesn't make Tremulous awesome and perfect. A game where the winning team is chosen randomly would also have a balanced win ratio, but is by no means fun.
(Also, admitting defeat is for the losers. I've made a comeback from being the last granger with no eggs several times, only due to practice and the usual incompetence in communication and coordination of movement/search in human team.)