It’s because of the canceled uploads.
The numbers may have changed by now, but:
A 29MB file will get cut up into 110x1MB pieces, those pieces will get uploaded to the nodes and uploads will be canceled after 80 of the pieces have been uploaded successfully.
If all uploads are about the same speed, then, when the 80 fastest are done, the remaining ones will be “almost done”, so you may upload 80-110MB for a 29MB file, so, the upload expansion factor is 2.75 - 3.79. This does not count any overhead with headers and stuff, so in reality the upload expansion factor would be a little bit higher than that.