Export JPEG changes file size

[KP8]

I've noticed that if I open a jpeg in Lightroom, look at it, make no changes, then use Lightroom to 'export' it to a different folder on my laptop, the file size changes significantly.  

For example, one photograph started at 7.8MB, and when I exported it to another folder it went down to 5.3MB.  What's going on??  

I have checked all the export settings - the 'Quality' slider is set at 100.  I can't see anything else that would suggest that I'm asking Lightroom to compress or change the image in any way at all.  All I'm doing is saving it again, in a second location.

Can anyone explain this please?  Does it mean my new version of the photo is going to be lower quality?
Thank you.

[dj_paige]

Please re-edit your original post to have a meaningful title.

Quality slider set to 100 could still cause compression of the image, that's how JPG works. You can't turn off the JPG compression when you export as JPG. Exporting as JPG always results in some loss of quality (although in many cases it is too small a loss in quality to be noticeable).

[JohanElzenga]

If you do not want Lightroom to open and save the JPEG again, then choose 'Export as Original' instead of 'Export as JPEG'. That will simply copy the original jpeg to the export location.