Why umap files are so big and how reduce its sise?

Hi,

One of the problems I have when I want to sync my job with other members of my staff is the huge size of umap files. Currently the one I am editing is 400MB and the terrain inside it is no more than 500 x 500 meters, with only a small subset inside populated with objects.

I was wondering why this files are so big if AFAIK they only store references to assets. I suppose they are the storage for lightmap textures because I have browse through the entire project folder and I do not see any folder related to lightmaps.

So if my supposition is right the more lightmaps we have (so say the more objects receiving shadows) the bigger the umap file is. Is this a correct supposition?

So my final question is: is there a way to reduce size in those files? For example, exporting lightmaps outside the umap files and making them a reference?