rather than upload to the album, can you try attaching to the post using the button with paperclip, want to see if it uploads it differently (it is the uploading that has increased the size)

I can't upload a jpg file with the paperclip, can I?

Test uploading 2.5Mb jpg
PhotoAlbum


As attachment

This image is 1736 kb
Same image with paperclip

yes, I think it is because it is silently converting them to 24bit png files (uncertain why, maybe a setting gone astray) which would also account for the missing exif information (png doesn't have any) possibly due to the rotate code for landscape image from mobile devices