I shoot with an A7iii (and I owned a D70s before), and while it's small, it's not uncontrollable or unbalanced for my use.

Bigger (mirrorless) cameras are bigger on purpose because you can fit more batteries inside and control heat better amongst other things, but I wouldn't want my A7iii to be the size of D70s or any DSLR, because it really hinders portability of these devices.

I agree Sony's APS-C cameras maybe miniaturized a bit too much, but A7/A9 series' size is perfect IMHO (and they manage their heat really well).