I highly doubt there is any correlation between save file size/complexity and a failure to upload to the cloud. There has to be another reason why it doesn't work for some people.
My save has all character slots and every single world slot full -- each character has a loaded back-pack, piggy bank and safe and every one of my worlds is pretty complex, with my main world having more built in it than any other console world I've ever seen (sky, surface completely covered with large detailed builds, pixel art and builds in the underground and a hellish city spanning the entire underworld.)
I have friends who can't upload to the cloud, and while they do have worlds that are built up, I've never had a problem uploading to the cloud and my save is massive.
Again, there has to be another reason. Can you link me to the thread about this?