Being able to play quietly while maintaining good tone quality is one of the (many) differences between professional and amateur musicians. It is always the orchestra's responsibility to play under the soloist(s). It doesn't matter if its a 15 piece show band playing under actors or a 110 piece symphony orchestra playing under a single violinist, if the soloist(s) can't be heard, its the orchestra's problem. Is playing soft and delicate while trying to ensure that 850 people in the audience can still hear you easy? No, it isn't, but it needs to be done: In the professional world anyway (I spent the last three years playing in a pro symphony orchestra).Don't know how many of you do play an instrument, but the play quieter things is easier said then done. You can do it, but you lose a lot of the tonal qualities of the instrument when you do that. With a stringed instrument, you get a completely different sound with played with a lighter bow, the amount of resonance and vibrato you can achieve becomes much less. This then "dulls" the sound. There is a reason that musicians want to play loud, it sounds better. Also, when you play louder, dynamics are able to come out more. With brass and woodwinds this become even more difficult.
Now, they should not be obscenely loud, but at the same time they are not going to be extremely quiet as well.
Obviously the OP isn't in the professional world and the goals of putting on a quality show and providing educational benefit to students sometimes (often) conflict with each other. The music director's desire to triple up on parts would indicate that (s)he is looking behind door number 2 while the OP wants door number 1. There is only so much you can do in those situations, especially if the OP is a student and the Music Director is a teacher. There are some good ideas in this thread for you to suggest through the appropriate channels, but at the end of the day, it will be what it will be. There are some things that are going to be out of your control and the only thing you can do is act professional and do the best job you can under the circumstances that you've been given. Keep your chin up, there is life after high school.