When attempting to download a world from Realms, it will immediately error out if there are lots of addons applied to the world.

From what I've tested, it might have something to do with the combined file size of the addons and not the number of them. I've tried removing different combinations of addons and there are not specific addons that cause the error, but removing a number of nonspecific addons works. If you remove a couple large addons the world becomes downloadable. If you remove several small addons the world becomes downloadable. A mix also works. It does not have to do with the world size itself and specifically has to do with addons. 

The world I'm using has:

59.87 MB in behavior packs

29.22 MB in resource packs