FBX file size increases after export

[matcarbo838P8]

Hello,

I am using FBX SDK 2020-2-1 and I encountered a problem while doing some test on a specific object that I cannot explain. If I just import and export without modifying anything obviously the file size does not change. But if I try making some modification on the vertex buffer without changing the total number of vertices, for example setting all coordinates to zero, then after exporting the file size increases up to four times the original size.
Shouldn’t the file size remain unchanged after such modification?

Thanks

[matcarbo838P8]

After further analysis I have found that the problem is in the morph targets:

In the original file only certain morph target values corresponding to the control points are saved in the “shape” buffer ( I guess these are the points whose morph target value is greater than zero ).

Then, after making some modification on the vertex buffer without changing the total number of vertices and exporting the file, dimension of the shape buffer becomes equal to the dimension of the control points buffer and therefore the file size significantly increases.

Is there any option available to preserve the “sparsity” pattern of the morph target?