It's just so hard to know how strong a race is just by looking at them, because WoW origins are so weird. For example, to look at them you'd think an Orc could instantly crush a human, even one of the beefy WoW humans. The Warcraft film really drove this point home. But WoW humans are derived from space robots by way of giant vkings. So I dunno - maybe they have the muscle density of a chimpanzee. Gnomes too, for that matter.
From a story perspective, I think we can safely say that a typical orc is considerably stronger than a typical human, but that's about it.