(Broken record...) I like how Fantasy Craft did their races. They include both Giant and Ogre as PC races. The biggest reason they work however, is that the core system removed the Size = better combat (Atk/Dam) problem that d20 had.
Large creature's STR being better than other races is represented more as the ability to carry/lift more rather than bonuses to attacks & damage. Thus a STR 14 giant fighter is equivalent in attack/damage modifiers as a human fighter with the same STR. However, the giant can lift/carry 5 times as much and has other benefits (and penalties) due to their size.