Probably only introduced the 24inch because of maximum pixel output limitation of M1. The 24inch is 4.5k and it supports one external display up to 6k.

A 27”+ would require to push at least 5k (possibly 6k) for the internal monitor + 6k for the external

For comparison the M1 Mac mini supports one 4K display and one 6k display. So it seems that this is close to the maximum for the M1 chip

It also means that for us with 5k2k monitors we will not get back our missing scaling options (at least not with M1)