Wrong Question. There is a Different Solution.
The real question you have here is "how do I make extra large creatures feasible," and you're looking for a solution for this by tweaking the square cube law.
That's a problem: you can't.
It's a geometric rule that relates area to volume. You can't change that without doing some really weird stuff physics-wise, like warping space.
But if your problem is how to make large creatures more feasible, there is an answer.
Take the strength to weight ratios of an ant, a cat and an elephant. The ant is super light, but can carry many times its own weight. The cat is heavier, but due to its muscle strength vs its body weight, can still jump many times its body length. The elephant however, can't even jump. Not that it's not strong, but the strength scales with the cross sectional area of the muscles. And the weight increases by the total volume of the creature.
So the strength doesn't decrease as much as mass decreases going smaller, but it also doesn't increase as much as mass increases going larger. This is a simple geometric and biological fact of life.
What you want to do here, is tweak the scale. If you were to make normal humans have strength to weight ratios of smaller animals, it would mean that the upper weight(or in this case, volume) for animals is much higher overall. Normally if you were to get too large, the bones or the muscles would no longer be able to support the weight.
So finally, to get to the point, the thing you really need is to change is: the mass to volume ratio.
Change the Mass to Volume Ratio
Simply put, you can have larger more complex creatures behaving as if they were at the scale of much smaller creatures if the matter itself that they are made of is lighter, but functions in the same way otherwise: the same strength of muscles, but the weight of the body is lighter.
This would mean your larger creatures could get truly BIG. Their muscles and bones would be able to handle much more mass. Your humans would be able to do things that would seem superhuman from our perspective. They would be able to lift much larger objects, and jump further, fall farther, with no ill effect. In fact, if you had melee weapons in this universe, they might look oversized by our standards, in order to make up for the lost mass. And thus you reach the realm of Final Fantasy physics.
The effect this would have on planets and the landscapes, I'm not entirely sure, but perhaps mountains wouldn't be compressed as much and slopes could be more extreme before crumbling. Perhaps the atmosphere would reach higher. The planets would have to be larger than Earth to have the same force of gravity. It would make life easier for flying creatures as well.
So there you go, I hope this helps you with your problem.
If anybody has major issues as to the feasibility of this solution, leave a comment!