Oops. Apologies, a faulty assumption.
Having seen midsize/midsized marked in the dictionary as chiefly North American, I extended it over middle-sized and then over what bhai said, and I ended up with:
'* I think in BrE it's much more common to say medium-sized '(much more common than in AmE),'' while what he was saying was,
'I think in BrE it's much more common to say medium-sized' (rather than middle-sized).