Exporting jpg to pdf: file size grows significantly
PDF supports JPEG encoding, so the size should not increase very much at all. It should be able to wrap some header info around the jpeg stream and call it a day.

Unfortunately, Preview is very good at viewing PDFs, but when writing PDFs tends to use a subset of PDF that is easy for it to parse, but not space-efficient. 

I'm sorry to say this, but you will need to look for a different program that can leave the JPEG alone and shove it in the stream.  Acrobat can do this, as can pdftex or convert from ImageMagick. There are probably others.