Hmm.. I'll give an opinion since I've played a few hundred games in the past month or so...
I honestly believe that in a anything from 2v2 to 8v8, it is rather even. Similar to what freak said, anything past this point, is completely insane and hard to control. ALTHOUGH, I've played quite a few 15v15+ games and I have seen aliens win a few times. In the end, it has a lot to do with habits of the team. Say all the humans do is camp. The aliens, should be attack them and suppressing them as much as possible at that time but you'll find that they're hesitant 3/4 of the time except for maybe a few exceptional players. It's not a good idea to run into a base with 15+ humans camping but it's also not good to wait inside of your base in groups to get hit by spam and flame. I've found that moving in groups of three of four in aliens (whether it be four tyrants, goons, whatever) that the odds of any human group really taking you down is reduced and when you take those groups and flank and suppress them in different directions, they tend to be pushed and stressed. Aliens though tend to be individualistic and are the opposite of this, including myself. I can't stand to be blocked by a basilisk who sits right behind me and refuses to move, while I'm getting sawed and flamed in front of me. I'm sure SOMEONE has seen me get pissed off at some nub that keeps sitting behind people and blocking them. One distinct time was when I spawned, when immediate tyrant, and went into a hall way to get blocked by two regular goons. Then two lucis came out of nowhere, whacked all of us and we all died.
Paragraphs a bit unorganized and sloppy but it's true for the most part.
