As seen at the end of Ant-Man and the Wasp, Ant-Man passes out and falls into the water when he increases his size considerably.
Hope says something like "He is too big!". Is there a correlation between Ant-Man's size and his stamina? Did I miss an explanation of this phenomenon during the movie?
I think this revolves around him using more energy to move his giant body, the energy he had remains the same but his body is bigger, that's why he gets tired more quickly.
One other explanation I heard is that his body was so big and the air molecules are so small that they didn't have a useful effect on him.
While the question of why growing to that size is tiring is not addressed, the fact that it is extremely draining does get mentioned earlier in the movie. When Hank, Hope and Scott meet with Bill Foster to request his help in tracking down the lab, Bill mentions that growing to the size seen on the news in Germany must have been exhausting, and Scott confirms that he slept for three days afterward.
According to Scott Lang, growing to enormous sizes is something any user of the Ant-Man or the Wasp suit needs to be really careful with. For a variety of reasons, it takes a big toll on the human body, especially the nervous system, making one feel dizzy and faint at a certain size threshold. That’s why he collapses into the water when he becomes too big at the end of Ant-Man and the Wasp.
Scott talks about this in his book, Look Out For The Little Guy!!, in the second chapter, REDOS & REDON’TS. He answers one of the FAAMQ (Frequently Asked Ant-Man Questions) by saying:
Q: I know you can shrink down to quantum size, but is there a limit to how big you can get?
A: To be perfectly honest, that’s one of those Hank Pym questions that I hate to ask him, because he takes everything as a critique: “Oh, so it doesn’t get big enough for you now? Why do you need to be bigger?”
But seriously, from what I’ve experienced, self-enlargement is something any user of the Ant-Man or the Wasp suit needs to be really careful with. For a variety of reasons, it takes a big toll on the human body. Or at least mine. For one thing, going big puts a strain on my nervous system. So if my size hits a certain threshold, I start to see things. Like, specifically, the ground coming toward me. Fast!
From the 17th chapter, FROM ANT TO MAN:
However, even these totally badass mecha add-ons can take a very heavy toll on the suit-wearer’s own personal reserve of strength. When I enter giant mode, I’m the opposite of Captain America: I can do that for, well, not all day.
In physics terms, it might simply be that the molecules and chemistry of an enlarged body can still somehow function as normal in enlarged state, but the electrical signals in the nervous system still abide by the usual physics of electricity. That is, even if the voltage requirements to operate muscles don't change, the current has to travel much farther to reach outer limbs, which means an inevitable loss of voltage due to electrical resistance.
It would be like doing a week's worth of hard labor condensed into a small period, but with only the fatigue of the nervous system (AKA mental fatigue) and none of the muscle or joint strain.
