.fbx – Too big blend file when importing mocap data in FBX

I would like to import motion capture data into blender.
My 40 FBX files are 200 MB in total.

Once imported into blender, my .blend file weighs 3.15Go.

I tried to clean the file, to remove the elements which are not important. but the file is still very heavy. (I’m using the Better fbx importer addon, but blender’s native fbx import feature does the same)

I don’t understand why. Have you ever had a similar problem?
How can I optimize my file to get a much smaller file.

Thank you in advance for your help,