How much and how little are logically equivalent, although in vernacular usage it is of course common to use how little when implying the amount is less than one might expect, and how much to imply the amount is more than one might expect.
replyEnglish 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.
replyIf 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.
Practical communication is not an exercise in logical deduction. The meaning of those terms is not equivalent for exactly the reason you noted.
replyhuman beings are not always perfect communicators, regrettably.
replyLuckily, other humans can 'clear things up' :)