Errata...
Encumbrance. I don't always use it, but when it crops up, I want it to make sense. Check out Carrying, Lifting, and Dragging on page 222 of the PH.
Encumbrance = Strength x10, x20, and x50 for a normal lord, heavy load (slowed), and maximum drag load (slowed and no difficult terrain) respectively.
Now apply those calculations to the heaviest and lightest PH races with average height and weight...
--Medium dragonborn; 6'5" tall; 270 pounds; 10 Strength.
--Small halfling; 4' tall; 80 pounds; 10 Strength.
Both of these races can carry exactly the same amount (100/200/500 lbs.). If that isn't nonsensical enough, the 10 Strength halfling (above) can carry 20 lbs. more than their own weight with the greatest of ease.
There's keeping the rules simple and then there's simplifying them too much. If encumbrance is so unimportant, then Wizards should at least base their calculations on character weight *and* Strength in some manner. Perhaps the printed ranges of character weight can be the base values for encumbrance.
If these encumbrance rules stand, not even a warhorse can carry a naked dragonborn of average weight more than 2 squares per move action (see Mounts and Transport, also on page 222 of the PH). In addition, while there is a +25% encumbrance modifier for quadrupeds, there are no multipliers for size.
I think encumbrance might of been one of those things that was overlooked in the final edit.