Well it's generally reported as a 25,000 capacity festival - although I accept there wouldn't be 25,000 on the hill all at once. They're hardly incomparable slots, though!
Clearly Khruangbin might not play, but I don't think any artist applies a bright line that way in any event. Sure, their average crowd size is likely to be broadly similar across a period of time, but they're always going to play some shows that are bigger / smaller than others, for any number of reasons - diary fit, dollar, vibe, profile, connections....
It's like saying Big Thief were never going to play Cardiff Uni last year on the basis they were headlining a 15k / 25k festival. But lo...
He's already headlined some of the biggest UK festivals with Reading and Leeds being biggest, and there's talk of him headlining Glastonbury too.
He's absolutely massive in the UK.