120 per side is problematic for several reasons:
- LAG. When the entire BG has 5,000-10,000 ms latency, it becomes impossible to kill vehicles.
- Combat mechanics. The META gets weird with this many players. For example, fire mages can do 2-3x the damage of any other class, simply by spamming living bomb.
- Class balance. Playing a clothie is already pretty terrible in Wintergrasp. More people means greater density of people, which means, for cloth-wearers, most of your time is spent dead, because it becomes increasingly difficult to avoid being in melee range of at least a one warrior who will demolish you.
- Vehicle power balance. Most importantly, with 120 players per team, vehicles behave as if they’re made of wet tissue paper. It’s simply not possible to win on offense with this many players on the opposing team, because the marginal benefit of each additional vehicle is so small. The team on offense probably needs on the order of 60-80 vehicles to have any chance of winning, which obviously can’t happen, and would be absurd even if it were possible. Even with 80 per side, vehicles may need a buff that allows their health/armor to scale based on the number of players present.