Explain how then?
Because he wouldn't be going there if they weren't doubling his salary. Everton's PL ceiling isn't all that much higher than Southampton's, if at all. And Southampton's arguably the better run club with a brighter future.
Everton were just taken over by a billionaire
NUFC is owned by a billionaire.
You think their owner is going to spend that much money on bringing in a manager and then just stop giving a fuck and not buy any players?
it doesn't mean he'll be successful doing so.
There is no fucking way Koeman is going to Everton because they're a "bigger club" than Southampton.