And indeed, to reduce terrain costs commensurately. Less space equals less terrain to cover it.
Agreed. I don't do 15mm a lot (apart from WWII stuff, 15mm minis are pretty hard to find in my neck of the woods), but when I do I can usually replace "inches" with "centimeters" and use the rules as written. That lets me turn a 3' square board into a 36 cm square board, which means I can play on a coffee table. Even games that require a 6'x4' board can be played on a 3'x2' surface with room to spare.
On the other hand, since most of my terrain is designed for 28mm stuff, it means that I mostly only have hills (BIG hills!) and forests (DEEP forests!) for these games.