The state uses lot size by soil type, where if I recall correctly, decent soils will yield about 4 bedrooms per acre. This document is more specific, with a lot sizing section starting on page 41.

https://www.des.nh.gov/sites/g/files/ehbemt341/files/documents/2020-01/Env-Wq%201000.pdf

Towns can - and often do require lots to be bigger than this minimum which is 30,000 sq. ft. for a relatively flat parcel with very good soil, which can get pretty tight with individual wells - each with a big protective radius, and individual septic systems. Historically some minimum lot sizes were established in order to minimize any contamination of wells from septic systems, which made sense when land was inexpensive, and it didn't cost so much to be extra careful.

But of course now (and probably also to some extent historically) towns with bigger lots tend to be more exclusive - which is a horrible reason to keep 2 or 3 acre minimums.

And besides that, individual homes tend to bring in a lot less in taxes than what most towns will have to spend for schools to support the new students. Which makes senior restricted housing more attractive than homes for families.

In any case, places that have town or city water, and waste treatment have much more potential for higher density. While there are some opportunities for improvements in smaller towns, but in most cases it will be a hard sell locally. And imposing new state regulations on towns is a slippery slope that has already limited the flexibility of local government in many ways.

In our town we did pass an ordinance allowing each single family home to have an accessory apartment (as long as the septic system is sized for the total number of bedrooms). And I'd love to see one tiny home as an alternative (or additional) option.