Upload & Sell: On
Just an off chance, are the smaller files cropped? Leading to the reduced pixel sizes?
Also, nothing wrong with only uploading 800px images, faster uploads afterall, but is there any reason to not just use smugmug's max image size to display setting? I.E. upload full rez images but limit the viewable size to 'Medium' or 'Large'?
I use the SM Lightroom plugin as I have found that uploading through Smugmug does something to the file and I'm not sure what. I have done a lot of testing with the New Smumug and I find that the best method is to publish using the plugin and u/l using 1000px for both the long and short at a 260 resolution. When upload full rex files, my results indicate that IQ suffers regardless of the viewable size (YMMV).
Why there is a difference in size as the OP states could be the upload process from Smugmug. I'm only speculating but the SM upload process may do something to the files in terms of size to optimize disk space on their end.