When we get to really big files, it's better to not have thousands
of small chunks, since we don't have a resume functionality if the
upload fails. Better to try upload less chunks even if those chunks
are bigger.
For example, with this change a 20GB file would go from 4000 chunks
of the default 5mb to 1000 chunks of the new 20mb size. Still a lot,
but perhaps more manageable.
This is somewhat experimental -- if we still don't see improvements
we can always change back.