Some time ago when we were booking flights, we couldn't get on a Thai Airways flight as they were full, so we booked with Singapore Airlines - it's in the Star Alliance group to which Thai Airways belongs.

When we got to checkin, the guy at the counter said my bag (26kg) was too heavy. I'd already given him my Royal Orchid card so I told him that as a Royal Orchid member I had a 30kg baggage allowance.
"You're allowed 30kg with Thai but with other airlines in Star Alliance group it's 20kg.That's policy"
He didn't impose any excess baggage but it's worth knowing about this.