Did you find an answer?

I was trying to reduce it last time, and couldn't.

Had to reduce the space between answer to save some space.

I've found this in the documentation :

One can also change spacing redefining the following dimensions (here are the default values):

\AMCinterIrep=0pt
\AMCinterBrep=.5ex
\AMCinterIquest=0pt
\AMCinterBquest=3ex

These dimensions are the vertical space between questions (quest) or answers (rep), in boxed mode (B, with \AMCBoxedAnswers or box package option) or standard mode (I).

I changed the space between the answers (boxes) using \AMCinterBrep, however, for the space between one question and its own answers wouldn't work.

I noticed that the \AMCinterBquest is for the answers and the next question, not the same question.

Maybe I am doing something wrong.

Did you type box with the package ?

\usepackage[box]{automultiplechoice}

Yes, I'm using that option.

\AMCinterBrep is the space between answers in boxed mode, but not between question and answers.
Maybe you can add a negative vertical space just before the choices environment. If this works, you can create a new environment shortchoices that inserts this negative space and then calls choices).