Huh that is weird - the models don't seem like they're too big for 25s.
Here's how it is, though - I think it's safe to say that most people and tournaments doing matched play measure from base? There's no official agreed upon base size for units (except when they come with a specific base size) but there is a general idea that they should go on an equivalent size to their squares - 25mm rounds for 20 mm squares, 32mm rounds for 25mm squares etc. I'm fine with that. If tournaments don't have specific guidelines they usually have a "don't game the system" clause, meaning don't be a ****** and put your larger infantry on smaller bases. In the context of the rules the tournament is going by, it's cheating. I'd be annoyed to see a unit of 20 Ard Boyz on 25mm rounds. They'd be getting a shitload of attacks in.
The new daemon bases are tricky - I believe they were on 25mm squares? Seems unfair to force players to rebase again, but if you want to play the tournament scene you've got to play by the tournament rules - TOs don't owe players anything. It's a private event, and if it's competitive they want to make sure that it's fair for all players.
It sucks, but if I were a new Khorne player using the 32s I got in the box and went up against an older Khorne player using 25s, I'd be annoyed at least.