Editing Image in PDF Seems to Cause File Size Bloating

I find that if I open a PDF in Acrobat Pro DC, select an image in the file, open it with Photoshop to edit/save it, then save the PDF, the file size grows way out of proportion.

For example a one-page file with one scanned image of sheet music on it: In Acrobat Pro DC, I select the image, Edit Using Photoshop, delete some stray marks, close/save, return to Acrobat. After saving the PDF, the file goes from about 250KB to about 2.2MB.

I further find that if I Save As the file, and either overwrite the old one, or create a new file, the Saved As one is back to a reasonable size. Simply saving seems to keep a lot of unnecessary information around. Or if it is necessary, what for?

The growth seems to occur during a plain-ol’ Save. If I Audit for Space Usage before saving the edited file, the space used by Images is roughly the same as it was before I edited the image (232KB before, 257KB after). However, after I Save in Acrobat Pro DC,  I grows to 1944KB. If I then Save As, and replace the file, Audit for Space Usage now says 202KB for Images.

Is this a bug or a feature?

Thanks.