I think it has been subject to errata, but (IIRC) the 4E encumbrance rules in PHB1 worked out in such a way that a dragon could not lift most animals. My memory is a bit fuzzy though; the group I usually play 4E with just ignores the RAW encumbrance rules, so I am not as familiar with them as I once was. I'm not really sure how those numbers were decided upon.
GURPS 4th Edition (the other game I play a lot) goes into a little more detail about why things are the way they are. I believe there was some research done into what the average person could lift over their head with one hand in one second, and then other numbers were extrapolated from that. Honestly, offhand, I forget exactly how it is explained. I just remember reading why Basic Lift was calculated differently in 4th Edition than it is in 3rd Edition.
Pathfinder -which I just started playing recently- follows the D&D 3.5 model, so whatever the justification for 3rd Edition's numbers were would apply to PF as well.