If you have a passing acquaintance with physics, and if you have the least knowledge of biological construction, you know already that giant creatures are flat out impossible, and that if they came into existence at the flick of a wand, they would collapse immediately under the weight of their own flesh.
There is a reason why insects or crustaceans are small, particularly on land. It is the nature of the exoskeleton, in which the structural integrity of the creature comprises an outer shell enveloping the meaty goodness. The bouyancy and pressurization of water will enable larger crustaceans to exist and retain their integrity, but the gentle reader will take note that one does not see four-pound lobsters clambering around, or 20-pound oysters or nestling upon dry land.
It is a simple mathematics problem. If we consider a beetle, for example. It is easy to find an example that might be one centimeter in length, and perhaps a total of one fifth of a cubic centimeter in size. If we suppose that the beetles mass is equal to one fifth of a gram as well, we can compare the beetle's length to its mass as a ratio of 5:1.
If we increase double the length of the beetle, we increase the overall surface of that beetle's shell by the square of that length. The larger beetle's skeleton is now four times larger than before. However, the mass of the body is cubed, and is now eight times larger that it was before. The larger beetle now weighs 1.6 grams. The ratio to its length to mass has changed dramatically, and is now 5:4.
As we continue to double the size of the beetle, the mass increases exponentially while the body size of the beetle increases only arithmetically. In other words, in very short order the beetle hits a size in which it cannot exist. This is why the largest beetles in the world are only 15 centimeters long.
It is no different with creatures with internal structures, like ourselves. If we double the height of a typical 200-lb. human, keeping all things in the same proportions, we find we have a giant that weighs 1,600 lbs ... with a far lesser percentage of that meat directly attached to the bones supporting the weight. Even if we propose that the bones are larger, and that the tendons supporting the muscles and attaching them to the bones are larger, we still find ourselves with massive amounts of flesh and organs which will simply tear free from the structure of the skeleton, reducing our giant a mass of gooey muck.
Now, what I am not saying is that there should be no giant monsters in D&D. Giants, along with huge crabs, ants the size of dogs, hornets the size of Volvos and so on are romantically fascinating things, and the game demands their existence. Rather, I am proposing that there must be some other justification for the presence of these huge versions of smaller, natural creatures. The obvious answer is magic.
We can suppose that there is some sort of 'magical integrity field' that enables the bones of a giant to support its massive size, or a rhinocerous beetle the size of a bus to scurry over the earth despite the comparison of its tiny legs to its necessary weight. Perhaps it is some sort of net, with invisible nodes that extend to each critical support point in the monster's body, sustaining that point and keeping it free of the inexorable pull of gravity. One might imagine having goggles that enable the viewer to see the magical 'tendons' extending throughout the giant creature's body.
Would this not, however, mean that if you chose to detect magic in the direction of a giant scorpion hiding in the bushes, the presence of the scorpion would be revealed? Shouldn't this be true with all such creatures? And if I should choose to dispel magic, shouldn't the body of the cyclops simply collapse, killing the creature? And what other ways might there be to magically affect his integrity field, once we have argued for its existence.
Which creatures would, and would not, have this field? A giant rat is no larger than a small dog. Certainly the reason there are no pig-sized rats in the real world is not because pig-sized creatures can't exist due to the skeletal-to-body mass ratio. So where does one draw the line, exactly. Where does "very large bear" end and "impossibly large bear" begin? When the bear is 1,500 lbs? 1,700 lbs? Do I need my Guinness book of records to tell me when the creature starts being 'magical?'
We could posit that large creatures are 'magical' but that they're a different kind of magical that isn't affected by the existing canon of mage spells. That seems like a bit of a hand wave to me, and in any case begs the question that if this magic integrity field exists, how come mages through the ages haven't researched to find spells that directly effect the field - never mind simply causing damage to the creatures, but rather simply obliterating the creature's ability to keep itself standing upon the earth's gravity well on its spindly little legs?
And while we're on the subject, perhaps the existence of giant creatures isn't regulated by a magical field? What if there's some other reason? I can't think of one and I'm open to suggestions. I know some of you out there will be thinking "smaller gravity well," which has long been the staple of science fiction novels. It won't work. At some point the limit is still met, and mass will still fail to hang off the bone. You've only changed the ratio limit.
The hand wave is the obvious solution. But are we really looking for solutions, or are we looking for new ways to tap into inconsistences that have been hand-waved to the point where we've forgotten a hand wave was necessary? Why shouldn't anti-giant creature magic be in some way be available? "A wizard did it" only preposes that a wizard can undo it.