It’s not just in movies. Think of ant and elephant

The strength of your muscles is based on how wide they are. Being longer doesn’t make them stronger.

As you get bigger, the cross-sectional area (width) of your muscle increases, but your weight increases more since you get longer too.

For example, if you become 2x bigger, that means 2x length, 2x width, and 2x height, and overall 8x heavier. However, your muscles, since length doesn’t matter, are only 4x stronger

Imagine sprinting. Usain Bolt runs the 100 m in about 10 seconds, and he does it with 41 strides, every time.

Now imagine giant Antman taking 41 strides in 10 seconds. Imagine the amount of distance his foot would have to move with each stride. Something like 100 m in less than a second. The amount of energy needed to do this would be absurd.

Another way to think about it is to imagine spinning around with a stick in your hand. You can easily turn once per second with a 6′ broom outstreched, making an arc about 12 feet across (about 37′ travel distance for the tip of the broom). Now imagine if you tripled the length of that broom. Your turning speed would drop dramatically, because it takes so much energy to turn. If you triple your 6′ broom to 18′, you are making the tip of that giant broom travel 113 feet per revolution. No way you can spin hard enough to get those speeds, so the time it takes you to spin a full revolution is gonna go up.

tl;dr It takes a fuckload of energy to move stuff that quickly.

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