First of all, are you sure you really need that? Have you calculated the memory footprint?
A small back-of-the-envelope calculation: A single mob and its state should fit into 100 byte of data. Let's give it a whole kByte, in case you are doing something extraordinary. When a cell has 1000 such entities, it requires a MByte. If your world is 100x100 cells, ...
One way you could solve this problem is not actually storing state on disk, but just setting up your generation code to use a seed for the random number generator, so it generates the same thing for a given area every time the area is generated, deterministically.
Then you just keep the 1000 or so most recently visited areas in memory. When areas are ...
Save it to the hard drive instead.
Saving large quantities of data in a non-volatile fashion is literally why hard drives were invented. If a player isn't interacting with it anymore, you won't need to rapidly access it, so why keep it in the RAM? Just save it to the your server's hard drive in a text file or something.
After trying several things it seems that it is a problem with version 3.7.1 of Monogame. I switched to Version 3.6 and the padding is gone. I will update this answer/question if I find the reason why 3.6 behaves differently then 3.7.1
For a project that we worked on, we had utilized grouping. It was one creature (with a model of many creatures), but had many hitboxes. When a hitbox HP became 0, it would divide the creature and create a second or third model.
Utilize low-poly models. We had to use this a long time ago for massive battles with multiple mobs.