It's weird.
I'm 5'2" and 185: my elasticated-waist jean-type trousers are a UK16 but my blouses, shirts and tops are UK20-UK24.
I too have no butt (comparatively) but above the notional waist, I'm the appliest of apples, so I suppose it makes sense.
Still, I've found a couple of tops that go over some skinnyish black trousers that look young and slim and oh my, is that me?
It's the quirks that keep us going!