Why Does Lightmapping make everything Super Bright?

Hello! I've been doing lightmapping for a while, and usually when it looks good it's by accident. I had to re-do my scene since I added a ton of assets, and aside from Unity crashing after 10 hours of light mapping, the results are terrible.

I have one directional light, the sun, which is set to 0.5 intensity. As you can see in the image on the left, it's not very bright in the scene. However, the image on the right shows the lightmapped ground and building, both of which are MUCH brighter than I expected them to be.

Why are the results of the light map not the same as the real-time lighting? Why are they so much brighter? Also, is there any way to keep unity from crashing after making a lightmap for a giant outdoor scene? It takes 10 hours to complete, so has to be done while sleeping, but always crashes, usually losing the data, when it's done.

Thanks!

Settings of my light: