You are probably missing a lot of basics.
Look up the following stuff:
What is a Texel.
What is Texel Density.
What is a Light UV Map.
What is Mip.
Once you know that, then graduate to complex things like:
How to build light-map UV with mips in mind
Once you know what’s up, then understanding why you got the result you got and why changing the resolution helps becomes a little-bit easier if not just logical…