What does "守望屁股" mean?

暴雪游戏守望先锋Overwatch。因为游戏里的女性角色屁股一个比一个圆，一个比一个大。所以这游戏也叫做守望屁股，或者屁股先锋

ENBlizzard's game Overwatch. Because the female characters' buttocks in the game are rounder and bigger than one another, the game is also called 'Butt Watch' or 'Butt Pioneer'.