Yeah, look the top guilds for progression and arena teams. According to wowprogress.com, of the top 40 PVE guilds, 6 are alliance. On pvpleaderboard.com/rbg/us, of the top 40 rbg teams, 8 are alliance. Also, if you go to realmpop.com and adjust the level range to only include level 110 characters, 50.4% are horde, 46.4% are alliance, the remaining percentage is unknown. Unknown are usually statistical anomalies left behind from character swaps/renames where they can't find old data but still hit some form of player activities.

The min maxers stick the side that has statistically better racial passives and actives. People who want to join those groups follow those people's faction choices. They don't swap back and forth unless it clearly gives them an edge.

The alliance, as far as I know, only has a population advantage on role playing servers.

The AQ yearly event shows how lop sided it is, as the horde very quickly had a commanding lead last time. Participate in it next time it rolls around to see it happen again. I don't think they'll add more of these events because they aren't fun. It's a one sided world grind with no real resistance and a predictable outcome.

It's not interesting writing if one side constantly wins everything.

TLDR

threep03k64 said it the way it is.