Think about any part of your body... say your arm. Now imagine scaling it up to double the size.

Your muscles are now double the length but also have to move a doubled distance. That changes nothing.

That same muscle (it's roughly a tube) has doubled it's diameter, so it 2² = 4 times as strong because it's cross-section is a circle and the surface of a circle is (d/2)² * pi.

But your whole arm has doubled it's size in all 3 directions. So it's volume/mass is now 2³= 8 times as high.

So in short: double your size and you are 2² = 4 times as strong, but you also have to move 2³ = 8 times the mass. That's the square/cube thing that makes just scaling up impossible.

PS: Yes, if you ever wondered how you were so incredible good at climbing things when you were a child... small children are much stronger than you compared to their own body weight.