I think that a bigger vocabulary isn't necessarily better - in fact, I think it leads to frustration by the game player, typing in all sorts of words that have *absolutely no bearing* on the game. The parser says 'yep, looks good!' but the game has no intention of handling them. Personally, to some degree, I appreciate it when a game comes back with 'I didn't understand that sentence' - it keeps me on track. For example, if there is no reason I need to 'climb' in a game, then the first time I try to climb something & the parsing fails I can then move on to something else, rather than keep trying to do something that is impossible.
You are right - it would save no appreciable space. It would speed up the parsing, but I'm sure not significantly.