English is a very context dependent language, so the phrases "surprised by how much" and "surprised by how little" actually have opposite denotations. Mechanically, sure, they're interchangeable, but using the "incorrect" form carries extra connotations. You've used the counterintuitive form, demanding the reader more carefully consider your meaning, which heavily implies you're making a point beyond what appears in the text.

If the second half of that reader engagement doesn't pay off, the sentence reads as clunky or incorrect, likely a mistake from a second language speaker.

Natural languages are not code and words can't be swapped arbitrarily. Word choice and construction carries meaning in and of itself, totally divorced from the actual words used.