That's not true at all. Foden is slight but he's strong, we saw that in the preseason game against United's first team where he was up against Herrera and Pogba and got in plenty of physical scraps with them. It was probably the biggest indicator in that game that he was ready, we all already knew he was skilful enough and smart enough for senior football, but he held his own against one of the biggest sides in the league.
At the u17 world cup as well the opposition couldn't knock him off the ball at all without blatantly fouling him, it was clear that he was stronger than a lot of the competitors, probably because he's playing with the first team every day and has a first team strength and conditioning program.
He's easily as strong as Sterling was at 18 when he started playing in the Premier League full time.