What makes Orchestras better?

As a learning composer, what is the true difference in writing for a quintet, a string orchestra, and full-blown orchestra. What is the difference in writing styles for the 3? I'm asking this because I feel like I'm not using the full potential of all the instrument (I'm writing a Requiem mass, and paused it so I could learn more about how to write). I was part of a group a few weeks ago. We had a full 200 persons chorus with a full orchestra. And It was riveting, and it left me wondering, how can I do something as powerful with my own writing. 

All-though I have a boat-load to learn in simple 2-part and 3-part and 4-part, I also want to have some fun. And writing a practice orchestral piece is next on my list. What is the order I should plan/create my music in. What specific techniques are useful for orchestral writing. How can I be like the cool kids with their orchestral pieces. 

The thing that separates the groups that you talk about is down to many factors. 

Some of these are the depth of sound and range of colour. Adding winds adds heaps, even when they are only doubling the strings. 

About your practice tips:

As I am still young (14) I can't over you solid steps, only my advice. Plan it like you would any other piece - I like to plan mine step be step - and think about what instruments you will use where. Doubling is not something that is thought about often, as beginning composers especially think it is too obvious. Remember, make the parts interesting, and don't just make the accompaniment long notes (unless you want that effect.) My final tip for you is to read Rimsky-Korsakov's Orchestration Manual, which you can find on IMSLP.

You can get a lot of learning out of copying what the big name composers have done.  Compose something.  Then go find the conductor's score to one of your favorite composer's pieces and copy what they did in their orchestration into the orchestration of your piece and use the playback feature on your composing software to listen to the result.  Pick a different big name piece by a different composer.  Copy what that composer did in their orchestration.  Compare the two versions to hear how the same music changes depending on what combinations of instruments are playing together.  It won't be as helpful as listening to a live orchestra, but it will give you a sense of what other people tend to do, and what sounds result.  If you can't find a full score to look at, you can make yourself a quick visual map by ear.  Get a piece of paper.  Listen to a famous piece.  Mark which instruments are playing every 30 seconds using color coded highlighters or markers.  Label which of them has the subject, counter-subject, where the B section starts, where the big climax occurs, etc.  You'll probably want to listen to the recording more than once, and to pause the music while you are making notes periodically, but the point is not to get every detail of the music on the paper, just to get a broad outline of how the composer used their trombones, or whatever it is you are curious about.