> You are trying to use the word that once you get the match result back, it discards the most number of words.

This isn't strictly true either. Two N-sized subsets of words from a common initial set might have completely different difficulty in reducing further, because in the worst case there might not be a valid guess which nicely spreads the remaining words out among the 243 possible outcomes for that guess.

Set-size is a good heuristic, but it's just that - a heuristic.