I don't really get why /u04/oradata/nbs/nbsrbs01.dbf is listed so many times, but...

Unless you have specified an OPTIMAL storage parameter, your RBS will not shrink automatically. During typical database operation, your RBS will grow to a certain size because you have a transaction that is that big. You can shrink it down, but the odds are that it will grow that large again. If you are getting "snapshot too old" messages or you can not complete some of the transactions, you probably need more RBS space. If not, leave it alone.