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".