Is there a tool to reduce file size of .exr (lightmaps) automatically?

I am using beast lightmapping tool to bake our models and so far it is working well. I have one concern that other than our terrain (which always have just 1 512x512 lightmap) the other assets are having around 112 lightmaps, which are not as efficient as the terrain. I can reduce their sizes through Unity Default Player Settings, but then while exporting a package, their file sizes are more than 500 mb. I think what I would need is a tool to convert rest of the lightmaps from 512x512 to 32x32 with an external filesize reduction tool or something like that. Do you know any ways to do this or is there anybody know if there is a tool exist?