> This is incorrect. The amount of material used is the third factor in a fair use test. There are other ways it comes up as well (e.g. damage calculations).

The 3rd factor of the fair use test deals with how substantial the copied material is in relation to the entire work. You can make a copy of 100% of the original work and have it still be fair use while in another situation copying 0.002% of an original work would not be fair use. The measurable amount (bytes, seconds, square inches) you are copying does not matter here. Instead the "importance" of what you are copying to your criticism is being defined in this prong.

For instance:

I am a movie buff and:

1. Make a copy of the entire movie of Citizen Kane 2. Remove the audio and replace it with a commentary track going over every framing/camera trick through the movie 3. Upload this to an educational youtube channel that teaches viewers how to shoot a movie

I would likely be allowed to use fair use as a defense to a copyright claim. Here, if an expert could say all 100% of Citizen Kane's film, contained relevant film techniques that I was actively commenting on, I would have copied the correct amount for my usage.

If I instead owned a movie review channel and I said "Citizen Kane is my favorite movie" and I displayed the entire movie after that I would likely not be using an appropriate amount of the film.

It's also important to note that fair use is a very narrow defense that covers a very limited subset of uses. It is not a generally applicable, or even a guideline, for how to skirt around copyrights.