How not to come off as boasting or arrogant, if you are one of the few active students in a class?
One good heuristic that you might use is to discriminate between two types of questions:


Questions with a clear "right answer"
Questions encouraging discussion


Questions with a clear answer are typically more about polling for whether people are tracking, and answering deprives somebody else of a chance to answer.  For a question encouraging discussion, however, your answer instead may break the ice and make it easier for other students to join in with answers of their own.  Answering discussion questions also benefits you more, since they are where your answers can demonstrate deeper understanding, if you have acquired it.

If you and your instructor both know you are doing well, then why not "step back" and leave most of the "right answer" questions for other students?  Especially if you let the instructor know that you have realized you are talking a lot and want to give space for others, it should not reflect badly on you in any way.