Download for shared album is smaller than original file

The downloaded file is usually always smaller that the original in physical size-on-disk terms because the download is in jpeg format, and some element of jpeg compression will be applied to that file (the effectiveness of which can also be affected by the actual image content). However, that's not the measure you should be using, you should be more concerned with the actual pixel dimensions of the downloaded file, which should be the same as the original. Is that the case?