But by canon Spock is very strong.
Mister Spock is much stronger than the ordinary human being.
Aroused, his great physical strength could kill
Kirk didn't say Spock was stronger because he lived in the gym.
This would seem to say that it is Spock not being a ordinary Human being that makes him so strong. Leonard Nimoy had a good build, but he wasn't extremely muscular. So what gave Spock his great physical strength would be his half Vulcan heritage and up-bringing. Coming from a slightly higher gravity planet, growing up there, would give Spock increased bone density and increased childhood muscle fiber. Vulcan's low atmospheric pressure would also give Spock an advantage in Earth sea level pressure, his blood would be better able to supply oxygen to his muscles, I alway though that Spock would have had to be careful to constantly breath somewhat shallow. His heart, used to moving his blood in a high gravity environment, would be better at moving oxygenated blood to his muscles in a one gravity environment.
However, spending many years in a (presumably) one gravity environment, even if his quarters were higher, Spock would not possess the muscle tone he would have if he had continued to live on Vulcan full time. Think of the muscle tone of Stonn and the headsman in Amok Time. Sarek, although more fully dressed, also looked a bit "beefy" in Journey to Babel.
If Vulcan possessed a one gravity surface gravity, Spock would have the same strength as a Human of the same size and muscle tone.