Being strong doesn't make you relaxed. Extra muscle mass and bone weight requires an increased amount of energy used in order to move, and even to just "be."
I believe that non-human mammals are not intellectually engaged in controlling their musculature, and so do not tense up. Even intelligent primates such as chimps and gorillas (who are mostly terrestrial, not arboreal) are a lot more comfortable in their skins than are humans. We think, therefore we tense.