It is small to prevent from having everything that is imported be a larger lightmap, especially if it’s not necessary. For instance if you import a small object that really only needs a 16 or 32 lightmap resolution but the default is 256 or 512 this is really overkill for it. It’s better to have to always scale up than to realize you need to go back and scale everything down.
Also, having a good layout for your UVs will allow you to get away with smaller lightmap resolutions around 64 and 128 for moderate sized objects.