Out of the ones I've been to none of them are particularly large.
Manchester Academy 3 is the smallest of the Manchester Academy venues.
The O2 academy at Oxford is a smallish venue. Depends which bit of the Electric Ballroom they're in in London as to how big it is. I assume they'll decide which bit based on ticket sales, that's what these places usually do.
That's the festival lineup, all the big acts are drowned out amongst the smaller crapper ones the way they've done the artist listings