The advent and popularity of disc brakes has made this more confusing, because many forks will handle at least two wheel sizes ...
And added suspension travel had added some complexity. If moving an older frame to a longer travel fork, it will change steering geometry some. But if say an air fork is set up with 1/3 sag, it could be almost the same nominal geometry as the previous spring fork with little sag.
Air forks can be quite light, in the 1600 gm range is doable for not terrible money in the used market.
Rigid forks can be heavy or light, stiff or springy.
So what frames are you looking at, and what is your end goal?