Smaller NetherPortals allows you to create 3 different sized portals than the standard 2×3 block one. You can now craft them smaller to fit into tighter locations and they have an overall more streamlined look.
The 1×3 is simply 1 block taller than the 1×2 and still provides that compact look but gives a bit more headroom if that’s what you’re looking for.
And the 2×2 is just the wider equivalent of the 1×2 giving you 2 blocks of height and width. All of these portals function exactly the same as the traditional portal but allow for just that bit of flexibility if you’d rather not have so much space taken up.