Comment by samatman

Let's work from the inside out. "Portion", that's a part of something, pro- portion means by, as, of, toward. The -ate make it an adjective, the dis- reverses the meaning.

So it is not based on a portion of the fee (a percentage of it). Disproportionate.

This is bad because it systematically penalizes small transactions, which a proportionate fee does not do.