The academy and Scotland U20 winger took the player of the match award and helped create Glasgow's opening two tries for Dan Lancaster and Jack Oliver.
With the help of Glasgow's strength and conditioning team, Watson has bulked up since last season and says his improved physique has given him more confidence on the pitch.
Watson told The Scotsman: "I don't know how many kilos I've put on since I joined the academy, but Luke [O'Dea], our S&C, and the nutritionists, have really helped. You try to push yourself in the gym and then off the pitch, getting into better habits with eating to try and get the most from our training.
"It's one of the things you don't really think about, but it does make it easier once you're on the pitch. If you can match somebody physically, they might be more skilful, but you'll be able to tackle them."
Over his time at Scotstoun, Franco Smith has shown he has no reservations about giving young players first team opportunities but, while Watson has put himself in the mix with his pre-season displays, he's not getting ahead of himself.
"I mean, it's obviously still just a pre-season game, and once you get into the URC, it's a different kettle of fish," he said. "But no, I feel confident enough to do it, especially with the guys around me. They help and make my job easier."