I use 1600 for main living, and 1200 in a nice "unfinished" poured wall walk out basement, with (insulated) concrete floor with no posts, as area for all storage, hobbies, crafts - kids roller skated, skateboarded, build forts, ping pong, foosball, TV, living room furniture, all winter long. I could even drive a car in. And it walks directly outside to an all sports freshwater lake. No additional taxes or insurance on 1200 b/c unfinished. You would not believe how inexpensive built new in the 2000s. People do make mistakes, in my opinion. Just watch how they buy motorhomes; same issue.
Not the parent comment but 1500sqft roughly gets you something in the range of an ordinary 2br 2ba (waving my hands vigorously on room dimensions, whether 2br 2ba is ordinary, etc). That's most people's "basic/ordinary housing needs".
So looking out from that position, people interested in another 1000sqft are usually thinking of hobbies, exercise equipment, shop space, media room, etc.