Scales, for a rough example your running a Pserver with 10,000 players requires paying 10 staff and 50k of Servers/Networks.
Your running a Blizz Server with 100,000 players now your paying for 100 staff and 500k of Servers/Networks.
so yes they have higher overheads but it Scales with Population.
and Pservers while collecting cash via “donations and P2W” i would be supprised if it generates more than the monthly Sub Blizz Charges