Move for Larger Creatures

[Lupo]

Originally Posted by Kuu

When designing larger monsters, does Move increase with size, a fraction of size, or as a square root of size?

Or should it be modeled as Enhanced Move, since a larger creature has more trouble turning around? If so, what is the ratio of Enhanced Move to size?

You seem to imply that there are some "biomechanical rules" in GURPS which "realistically" determine the relationship between move and size.

There aren't such rules, luckily...

Originally Posted by Kuu

The reason I'm asking this is because I notice that, in many movies and fictional stories, larger creatures are much more clumsier and slower than smaller ones. This probably has to do with the square-cube law and encumberance, but I don't know the extent to which it affects a large creature.

You are getting this backward. In fictional stories, larger creatures are slow because authors and audiences think it is appropriate for them to be slow.
In your fictional story (e.g., your campaign), you should feel free to make your "large creatures" as slow as you like...

For example, how fast would a 40-yard worm-like creature (with ST 200, BL 8000, and weight 500 tons, that, if 2 yards long, would have Move 1) be able to move?

Exactly 27,2 mph.
No, really, who knows? And, most of all, who really cares?
40-yard worms are quite clearly "fantastic" or "superscience" creatures... what is the point of exactly determining their weight in lbs / speed in y/s? Such creatures should move at the speed of plot :)