Blacksmiths are known for being brawny folk -- hammering pieces of metal together is not easy work after all. But in World of Warcraft, even the smallest gnome or scrawniest elf can be a great blacksmith. Azeroth is a land where even the smallest people can wield the biggest of axes, so it would follow that they could craft them too, as well as any other sort of armor or weapon that they could imagine.
Typically, however, even in Azeroth, blacksmiths are, by and large, members of a class that can use plate mail and heavy weapons, such as a warrior, a death knight, or a paladin, just as tailors are usually spellcasters of some kind. So even if a blacksmith appears scrawny on the outside, he or she is very likely still quite brawny on the inside. Underneath that elf's pretty skin are muscles of steel!
Being a blacksmith implies a state of mind as much as it does a state of body, however. Working with metals is not something for the light hearted. The weight, the heat, and all the soot are not for people who like to keep their clothes clean at all times, for instance. It's also not a very socially-oriented profession, requiring long hours spent hammering away at something until it reaches perfection, often using lots of material in the learning process before you finally get one right. Blacksmiths of lore tend to be patient and hardy people, tempered and perfected by their work, like good, hard steel.