system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] How does Ant Man revert to his exact default size?

Maybe I missed it somewhere in the movies, but is there a "default size button" or could Ant Man presumably be 107% or 97.3% his normal size?
gpt
Pym particles operate similar to atomic orbital tracks in that there are a set number of stable sizes.  It takes the active application of the particles to flip you up and over the low stability sizes until you drop back into one of the stable positions.

Your natural, normal height is the most stable track.  Growing or shrinking you bounce along multiple stable points until you get to your desired height.  That's why it looks like a series of different-sized ghost images when you observe it.

The increments are larger than just a few percentage points plus or minus, so 107% would likely be a low-stability point and require active expenditure of Pym particles to maintain.