BSP Size
I asked IRC if a large number of visleafs could be responsible for the large filesize; someone (Mevious, I think? not 100% sure) suggested that it was a possibility. The rest of my post summarises the other remarks the IRC channel made. I was trying to help identify what was causing his bloated BSPs, as opposed to saying "these things will bloat up your map".
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In theory an increased number of visleaves could increase the size of a BSP. But it would be negligible compared to lightmap data. Visleaf tree data is very small compared to what is basically a texture.
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Hint+skip brushes and areaportals are great for optimizing framerate (when used correctly), but I've never heard of them being used to lower bsp size. Visclusters speed up compile time by telling the compiler that all the visleafs inside the cluster can see each other, but again this data must be stored in the bsp, and I don't see how it would lower the bsp size. On the other hand, maybe I'm just oversimplifying everything and these actually are factors. If so, please provide source material so that I can get educated. 
I think the biggest impacts you can have are:
1. Reduce the overall number of drawn faces in the map (Edit: this helps mostly due to removing lightmaps (#2))
2. Optimize lightmap scales (which it sounds like you are already working on)
3. Lower the quality and count of cubemaps (which you haven't added yet)
I never said they reduced the map size. I was saying to him that he shouldn't just make things func_details to reduce the number of visleaves.
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Visclusters don't reduce visleaves either (assuming you mean func_viscluster).
The filesize went down dramatically when I added hints and skips, and reduced the lightmaps of everything (except for the water, as I've already said), and I replaced clusters of brushes with just one, and added an overlay or something.
But yeah, my entry is coming along now :3 I just hope I get it finished in time
But yeah, my entry is coming along now :3 I just hope I get it finished in time
You probably don't want to do hint + skip until you're pretty much done with the map. Just my $0.02
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Dis true. You might want to edit the layout later or something.
Naah, Area 1 and 2 are finished completely now :3