Thanks a lot ! :)
" I don't understand how you get your instruments to sound so light but so full at the same time."
Actually, I love playing with contrast, putting a lot of very different-sounding sections in my tracks, with some that are more heavy-sounding that others. In order to achieve that, I of course play a lot with the dynamics of each instrument and the characteristics of each articulation (using quieter strings in quieter passages, with less short notes, this kind of thing), but I also try to fill a bit more the frequencies when I'm writing an epic part than when I'm writing some smaller passages like at 2:10.
Choosing your sounds can also help a lot - an example I like is how a choir will give different colours depending on the vowel used for your legato/sustained passages. I tend to see the "Aaah" and "Oooh" vowels as what you want if you're trying to make something either epic or more "spiritual", the "Mmm" vowel makes for a very mellow sound (First Flight, the part at 1:20) that works perfectly well as soon as you try to make something sound more "emotional"... When you think of all the possibilities, when taking into account all the instruments and articulations available, that makes a pretty wide palette already, but layering different instruments add even more choices.
To be honest, there are some parts I find quite empty now, and surprisingly, it's the more epic ones, but on the other hand they still work pretty well, so maybe that's not too much of an issue, for once. I guess it works because there's also a lot of movement - that's what I was aiming for, something reflecting the agility of the character. But there isn't any sustained notes for example in the last epic part (apart from the choir, but it's more marcato notes than actual sustains), and it's probably a bit poor in terms of harmony.
And yeah, I tend to think music like a story - I actually try to write stories through music, I just love working this way when I'm not writing for a specific project, it helps when you're trying to find which structure you want for your piece. And sometimes, you can come up with some interesting ideas as well thanks to the story you're working with - that was the case with the part at 1:30, which I still love. This darker part was actually inspired by the look of the character's weapon, and it influenced everything that follows - even the way the last epic part sounds... well, epic, but neutral at the same time, not too heroic or dramatic - because the character himself comes back as an hero, but he knows that behind the looks, the story is somewhat darker.
Another thing I love with this process, too, is that since you're creating a story with music, the story can be somewhat vague, and the listener can imagine the details himself. You put the basic plot structure and events in place, and let the listener imagine the rest, putting him in his own creative process.
Sorry, I wrote a long reply, I hope you don't mind. xD