At some point I want to rewire my garage. I'm wondering if there's room in the conduit to run a 60A circuit to the garage. I would use the existing conduit up to the hot tub subpanel, then run a new conduit out the other side of the sub to the garage.

Just guessing, but the conduit is maybe 1-inch. Anyone have an opinion on whether there's enough room? Thanks.

There are three problems.

1. For THWN @50 amp they probably used #8 wires, you can technically fit 9 in there, but
2. For 60 amp you need #6. Further with that many current carrying conductors in the same conduit you need to derate. Either way it won't fit (and you #8 may not be sized big enough).
3. If there are 3-4 #8 wires in 1" conduit, it's going to be a bear to pull more in there anyhow.
4. (ok I lied, four things), if by "subpanel" you mean one of those spapack things (essentially a disconnect with a GFCI built into it) there's not a whole lot of room inside it for pulling through extra conductors.

On the other hand, unless you need it for protection of physical damage, you may not even need conduit. It's required for the pool/spa, but you can possibly get away with suitably sized cable.