Metallica isn't bigger than Maiden, in Europe and South America.
In my country, both bands are capable of selling out 30-40k stadium seats if those bands don't play in neighboring countries on the same tour. The stadiums in Zagreb and Split can hold about 65.000 each in concert configuration. That number can only be sold out if we're talking mainstream music only. Examples would be U2 in ZG and EDM festival in Split.
These are the only two metal bands capable of selling out an arena in Croatia without any special thrills involved, like one of a kind tours and such. Simply put, on a new album tour, both bands can count on playing to 10-12k people. Judas priest can't, they've cancelled club gigs here in past.
Metallica can sell out US stadium tour because they're considered a big arena domestic US band. They play on US television shows, superbowls and such, they're a part of mainstream American culture. Metallica is mainstream in the US in a way, and should be taken as such when talking about target audiences.