Mine's very happy in a 16g. He's always active and exploring. The only thing I'd suggest is make sure it's a long tank vs. a show tank so that he'll have an easier time getting to the surface. I have a 16 tall and sometimes Tango overextends himself when hunting and then he has to race to the top for air.
probably the only time I would think a betta would be uncomfortable in a big tank is if they don't have enough hiding places so they feel vulnerable or if they have an illness or disease that makes them hard to swim around (and in which case wouldn't NEED a big space just a small one of course)
I think one of the common conflicts with keeping a betta in a large tank is that often the tank is not entirely devoted to the betta. And tankmates well-suited to a 55g (cichlids, rainbows, tetras, etc) are more often than not very iffy roomies for bettas.