>>298290840
Theres often Rush servers populated around this time of day. I've seen a high popcap S&D server occasionally full too.

>>298293269
Because every model, texture and sound effect in the map is packaged into it. Maps ported from Source games specifically have their BSP geometry converted into hundreds/thousands of models depending how modular the original map was. Baked lighting also means lightmaps get created for each model bloating it further. 20MB sounds like a pipe dream unless you want blocky GoldSrc style levels with cars crudely made from brushes and minimal textures as a simple map with no custom assets is already 5MB. 
You also get people who download a bunch of high poly shit with complex materials from the Unreal asset store and either slap it into a map with no concern for the filesize or they just forget to remove the unused asset, which of course gets packaged in with everything else.