Every major planet featured at least one grand arena.
It was a necessity. As long as more than a billion people lived on any given planet, the demand for exciting duels and competitive matches reached a critical threshold. Either the governing bodies gave in and acquiesced to constructing a large arena, or else the citizens would make sure to elevate someone else in power who satisfied their craving for the best and most exciting action between mechs!
Larger arenas provided numerous advantages over smaller ones. They could accommodate more people, for one. They also provided more space for mechs to maneuver and use any terrain features if the competition format included this variable.