I always try to vet the land value, and subtract it. The balance sans any pool, outbuilding, etc. is the "house" value. Then $/SF makes more sense between equals (in condition, age, quality) leaving only "size" to contend with. agree w/ above...
In fact, we use a flat line X dollars per SF for excess SF but in reality it could be an exponential scale where larger usually means more bathrooms, more appliances, etc. So the larger the SF difference in the homes, the higher change in value with more "things" should be offsetting the economy of scale of mere size.