why when export raw to jpeg somehow it decompress the file

This is normal and the way it is. This is what jpeg compression does.

Jpeg uses very aggressive data compression to reduce file size on disk. When you reopen a jpeg, it is decompressed back to original size. But not the original state - jpeg is a destructive file format and that destruction is irreversible and cumulative.

Jpeg compression is extremely effective and can shrink a file down to 1-5 % of original size. That's why the format is still around, despite the serious drawbacks.

If you want to see the real and native file size, save to PSD or TIFF.

A raw file, on the other hand, is also smaller than PSD/TIFF, but for entirely different reasons. It is not compressed, but since it is a single channel instead of three channels, that's already a third of the size. Bit depth also determines file size. A raw file is usually 14 bits, so smaller than a 16 bit grayscale file.

In short - there is absolutely nothing wrong here and no problem to "fix".