system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I've heard building mechas is really difficult but there is no problem in building small humanoid robots (Boston Dynamics). So can't we just upscale them and make mechas this way?
gpt
Square-Cube law is a bitch.

If you take a human, and increase them in volume so that length width and height are all 2x bigger, they will be 8 times the volume, but the area of their footprint will only be 4 times as big. The surface area of an object scales squared, while the volume scales by its cube. 

This means that the weight per square inch of foot will increase significantly.

There are other similar reasons why things don't just scale up, but most come back to square cube law in some form.