[DTV]

I thought they had designed the new sets based on the smallest available space.

I think this is semi-true, in that the base set seems to have been designed to fit Birmingham, the smallest 'normal' studio. But with some of the weirder shaped or sized ones, there seems to have been minor alterations to the design - Hull's set isn't quite as 'deep', as the studio is very long and narrow, and Tunbridge Wells' isn't quite as high, due to the lower ceiling. It's 'one size fits all', up to a point.