Can't download a copy of my realm

Every time I try to download my realm it stops at 620.0mb

What I expected to happen was...:
To be able to download my realm so I have a back up, as I want to downgrade from a 10 people realm to a 2 people realm, but want to keep the world I have created, which the only way to do is download a copy.

What actually happened was...:
The download stops every time at 620.0mb, no error message it just stops and does not restart, even by leaving it overnight.

Steps to Reproduce:
1. Open minecraft, and click on the pencil button next to my realm 
2. Click on my realm a press edit world
3. In game settings, go right to the button and click on download world

4.  Tge download begins but always stops at 620.0mb