When we say something has grown proportionately , we mean all the dimensions have increased by the same amount. It might be clearer to say all the dimensions have grown in the same proportion or by the same amount. The same is the case with its muscles. But look closely at the cube shaped guy. It got 10 times taller, but 1000 times heavier.
Weight changes a lot faster than height.
So it is with all animals if all dimensions change in the same proportion. So here is the bottom line. See what's happening. Our creation is getting stronger, but his strength is not increasing as fast as his weight. There is nothing to be done about it.