If you plan to add racial maximums to the game for the sake of realism, you may want to consider just what is needed to be realistic.
For instance, compare a human and a halfling. Lets assume same "shape", but the human is twice the height. Since volume is the product of three dimensions, and mass is proportional to volume, that means the human is 8 times the mass.
Strength will roughly scale as the cross sectional area of the muscles, which in turn should scale with the square of the height (again, assuming the same shape). That means that a human built identically to a halfling should be 4 times as strong. A +2 advantage for the human relative to the halfling isn't anywhere close to large enough!
Quickness of movement (ie. dexterity) should scale as strength/mass, so the halfling gets an advantage of a factor of 2 here. Factor of 2, not +2.
I submit that the structure of D&D simply does not allow this difference to be realistically contained within the rules in a fun way. For similar reasons there are no rules to support sexual dimorphism. After all, it is supremely silly to go after a dragon with a sword for ANY humanoid, and yet we pretend that this is OKAY within the context of the game. If a human male can do it, why not a halfling or a human female. The scale of that deed is already absurd enough that it is silly to try to draw distinctions between the various tiny (compared to dragon) creatures that will encounter it.
