The BG3 map itself is not terribly big. Breaking it down into small areas would make it claustrofibbically small.
If BG had come out in the last ten years, it probably wouldn't be so fragmented into small areas.
The map would most likely be more compact then.
However, the capabilities of computers have grown significantly since then.
Over time, most games began to move away from small areas. If you look at the newer high-budget RPGs, virtually all of them have either one compact map or several smaller (but still large) areas.
It looks like BG3 is definitely going in the other direction.
