You can understand it like this:
This is an example of separating an object, I'm going to draw a luxury table from the French court, consisting of a large table board and 6 table legs, the large table board is 1MB, each table leg is 1MB. if you export the OBJ directly, then naturally it's 7MB. or you split it up and lay it out in a SH3D file 1 large table board plus 6 table legs. You save the file and see it's 2MB because those 6 table legs are the same and add up to a 1MB.
The same big tree why take up a lot of file space, because he has thousands of leaves, each leaf is very large, let's break it down, a trunk plus thousands of the same leaves. 0.5MB (this is a bit exaggerated)