My (non-expert) assumption... was/is that a 'reworked' JPG cud not be 'better' than the original.
After all, after (each) saving, some details are gone.
The thing is, as far as the program knows you're saving a brand new image with incredible detail, or maybe you added a lot of highly detailed stuff to the pic. The program isn't psychic, it relies on the user to decide such things.
That being said, if you check the "estimate quality" box on that dialog, the program will do it's best to guess, although it will still be somewhat larger than the original.
What you should do is this: Open the original image, do nothing to it, then go file->export & select the JPG tab. While on this box, move the slider until the output filesize matches that of the original file (or reasonably close to it). Now keep that percentage in mind & go to the normal save dialog & set it's slider to the same percentage. Now save a copy of the unedited pic & you should see that it's the same filesize.
Once you have that done, make your edits & save the edited pic with the same settings you just used. NOW compare the filesize of your edited pic against that of the original.
Lastly, keep in mind that your edited pics will almost certainly be of a somewhat different size than the original (although 2x is way more than normal). Adjusting levels can change the ammount of data in the image (ie by lightening some shades of grey until they are equal to the white elements of the pic). Even if the ammount of actual data doesn't change, the way that it is positioned in the pic can alter the efficiency of the compression alogrithms (ie, if you wound up sharpening some edges, those won't compess as efficiently as more blurred edges will).