Basically, it's the law of bio-physics that says we can't have real-life giantesses made out of normal human flesh-and-bone.

To try to explain it quickly...

If you double the height of a living thing (doubling the linear dimensions), then while the height goes up by 2, the volume, and therefore the mass goes up by 2x2x2=8. BUT, the cross-section of the bones and joints only goes up by 2x2=4.

So, if you take 5-foot-6 woman, hit her with a growth ray and make her 11 feet tall, her weight goes up by a factor of 8, but the cross section area goes up by 4, and her bones and joints have to support twice as much weight per square inch.
This is why real life people with gigantism have bone and joint problems.

now, take that same woman and make her 55 feet tall. Height went up by factor of 10. Weight went up by 10x10x10=1000. But weight-supporting cross-section went up by 10x10=100. She's going to have to support 10 times as much weight per square inch of area on every bone and joint.

as you increase linear dimensions, the area goes up by SQUARED, and the weight (based on volume), goes up by CUBED. Thus, the Square-Cube Law.

This impact biomechanics quite a bit for big animals. Look at big, bulky animals like elephants and rhinos. Very wide, stubby legs. Giraffes are tall, but rail thin to reduce the weight needed to be supported. Horses, with relatively thin legs for their size get broken leg bones more often than say, dogs.

(why, yes, I was a physics major in college, why do you ask?)

here's a good blog entry with some humorous explainations of why we can't have real life giantesses, with pictures even!
[link]