aramis erak
Legend
the flaw there is than bone weight capacity goes up with the cross-sectional area, which is a square function of the size multiplier... the proportions must needs change, lest they begin to have the bone-lack-of-strength issue of severe cases of Marfan Syndrome and certain other giantisms.The main thing to know is, for objects that are similar aside from scale, volume goes up as the cube of height. So, a 12-foot tall giant, if he's made of the same stuff as a human, would weigh eight times as much as a 6-foot tall man.
So, let's say m and h are the mass and height of one creature, and M and H are the mass and height of another creature with the same build. And assume both creatures have the same density (not a good assumption from an engineering point of view, but if you go down that road you quickly conclude that giants are impossible).
Then what you have is:
(M/m) = (H/h)^3
or equivalently:
M = m * (H/h)^3
An 9-foot tall giant, for example would weigh roughly 3.4 times as much as a 6-foot tall human.
Then, if you really want to get into detail, consider the creature's build. For example, giants are depicted as more stocky than humans. You have to put a number on how much more, from the point of view of the thickness of their limbs and trunk. Square that to get the increase in cross-sectional area, and hence the increase in volume for a certain height. A truly massive giant (built like, say, a dwarf), for example, might have limbs 50% thicker than a normally-proportioned human at the same scale. This would make him 225% (1.5 squared) as heavy. For our 9-foot tall giant, that'd come out to about 200 * 3.4 * 2.25 = 1520 lbs.
The numbers for giants in the Monster Manual (cited above) are mostly in line for human proportions and density at the given height.
The same general rules of proportion apply to any body shape, whether human or whatever. You just need a good baseline, which may not always exist (for example, nothing in the real world is shaped like a dragon). Taking your dire wolf question, though, a wolf is probably a reasonable approximation for a lot of quadrupedal predators, though. An adult male grey wolf weighs about 90 lbs, and is 5.5 to 6 feet long, and about 2.5 feet high at the shoulder. This implies that the Monster Manual listing (9 feet long, 800 lbs.) probably doesn't include the tail, and represents a wolf about 5 feet high at the shoulder.
Rolemaster Companion had a decent chunk on how to figure it, accounting somewhat for needed size changes of bone...