Photoshop Elements :: Why Do File Sizes Change / When Use Process Multiple Images To Add Watermarks

I'm using Elements 11. In order to add watermarks to many JPG pics at once, I use the function "Process Multiple Files".
 
I select a source folder and a destination folder, and adds a three digit serial number to each file. I do NOT tick the checkbox marked "Change picture size". After that I define the watermark I want printed on my pics and hits OK. All the files in the source folder are processed and saved, with a new name in the destination folder. Fair and square.
 
But. The file size of the new file is heavily reduced, compared to the original. It goes from 10 MB down to 500 KB (in general). Why is that? Is there any way I can prevent it?