system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why there is a limit on growing?

Why can't humans/plants/animals grow further than what they usually grow to?
gpt
Well, bad things start happening when parts of your growth become uncontrolled.
Acromegaly and gigantism are both health conditions which result from your growth hormones essentially never turning off. This leads to all sorts of weird and bad consequences such as heart failure, kidney failure, severe headache, increased odds of infection and metabolic disorders, skeletal problems, etc. To be fair, some of these results are just because too much growth hormone can do some weird shit to your body. However, a lot of them, such as the issues with the skeletal system and circulatory system, are simply because that level of growth causes problems such as thickening of the heart wall, or simple inability of the body to support something that damn BIG.

If you are wondering "well, what if we just fixed all of those problems? Then what?" This brings us to another thing, the square-cube law. Essentially, when you scale things up proportionally, the volume is proportional to the cube of the original volume, while the surface area is proportional to the square of the original volume. In more simple terms, when you make things bigger, the muscles and bones are relatively weaker compared to the sheer bulk of the animal. This is why, when you look at the skeleton of an elephant, it doesn't just look like a scaled-up mouse: the bones must be much more thicker in comparison to the overall size in order to support its mass. (Google "allometry" for a more in-depth explanation.)

So, essentially, in order to get healthy, viable 'giant' versions of humans (or any other animal), you would need to make drastic changes to their basic anatomy in order to even ensure that they would survive. 

(Side note: this is why you will never see ants the size of skyscrapers. Also, underwater animals have an easier time coping with this because the buoyancy of water somewhat negates the effect of gravity. This is why you can see incredibly large underwater animals such as blue whales.)