Hash indexes are currently (9.6) a little bit larger than btree indexes [1]. There seems to be no space improvement yet committed for postgres 10. But stay tuned, maybe somebody will do it soneday. Hash indexes have not been recommended for years because they have not been crashsafe and therefore are not really that optimized in every term yet (size, query planner, ...).

[1] http://blog.andrebarbosa.co/hash-indexes-on-postgres/