I was actually thinking about humanity in the series earlier, and I thought the reason humanity seems pretty dominating in the Mass Effect universe could be due to capitalism. The other species excluding the Volus probably aren't as competitive. A lot of the gear is human made, even for non-humans, by human manufacturers. It would make sense for humanity to capitalise on opportunities to profit off of Krogan mercs and human individuals to even pursue merc work themselves, something the other council races would be less inclined to do so.
I still think humanity is a bit too numerous on the citadel, especially in the tower. We also don't know population numbers of the other species.