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.
