The reason is rather simple: It isn't the number of territories that increases the file size, it's the number of nodes and images. If you look at a 3000 territory map, you will often see it is quite minimalistic. Take hex earth for example https://www.warzone.com/Map/22530-Hex-Earth
. Each territory is just six nodes in size, and there is no added artwork.
Your territories are teeming with nodes. Let's take this one for example:
I counted over 100 nodes on this one territory alone. And this is one of your less detailed territories as well.
Also, each mountain (which is very detailed) adds about 60 nodes.
Overall, you add tons of detail, and as a result, you end up breaking the byte limit. In the end, you are just going to have to either settle for a smaller map, or reduce the detail of the territories dramatically.
Edited 6/17/2020 00:58:52