10.1: hash index size exploding on vacuum full analyze

I've some tables that'll never grow so I decided to replace a big index
with one with a fillfactor of 100. That went well. The index shrunk to
280GB. I then did a vacuum full analyze on the table to get rid of any
cruft (as the table will be static for a long time and then only deletes
will happen) and the index exploded to 701GB. When it was created with
fillfactor 90 (organically by filling the table) the index was 309GB.

I would've expected the index size to, at worst, remain constant rather
than explode. Am I wrong or is this a bug?

PostgreSQL is v10.1. Original index created on v10.0.