Combine JPG into PDF, size increases

I have Adobe Acrobat Pro XI

I have scanned pages in jpg format.

I start "Combine Files into Single PDF", add the jpgs and start it.

Unfortunately the file size increases: JPGs 6,28 MB PDF 7,42 -> 18% more

If I change the options in "Combine Files into Single PDF" (Smaller File Size, Default File Size, Larger File Size) it has no effect!

If I go to Tools->Preflight->Browse Internal Structure->Page/Resources/Length:  all the values have increased compared to the jpgs. The difference of each page is more or less like the total 18% difference.

and the pictures look worse in the pdf, text is not as sharp.

What I'd like to do is pretty simple: I want to combine the jpgs which are scanned pages and preserve optimum quality.