Here's a list of some files that make up Sun and Moon's data, which should explain the size. New features don't take up that much space. The biggest file is the Pokemon models file.
If it has a (?), it means I'm unsure that's what the file actually is. I'm not an expert on this knowledge either, this is mostly assumptions based on size, the demo GARC set, and what pk3DS tells me.
4 Main Folders
a/0/ - 1.94 GB
a/1/ - 243 MB
a/2/ - 168 MB
a/3/ - 17.1 MB
2.36 GB
In Detail
a/0/1/1 - Move Stats - 31.0 KB
a/0/1/2 - Egg Move Data - 32.0 KB
a/0/1/3 - Levelup Move Data - 84.2 KB
a/0/1/4 - Evolution Data - 78.9 KB
a/0/1/7 - Pokemon Stat Data - 146 KB
a/0/1/9 - Item Stats - 50.4 KB
a/0/3/ - Gametext for all languages - 20 MB
a/0/4/ - Storytext for all languages - 13 MB
a/0/6/7 - Battle UI (?) 22.5 MB
a/0/7/3 - Common UI Elements (?) 20.5 MB
a/0/7/7 - Zone Data - 8.65 KB
a/0/8/1 - Battle Arena Models (?) - 9.43 MB
a/0/8/2 - Encounter Data/NPC Model Data/Map Texture Data (All one file, I think) - 332 MB
a/0/8/6 - Map Models - 61.2 MB
a/0/8/7 - Overworld Models - 143 MB
a/0/9/1 - World Data - 52.6 KB
a/0/9/4 - Pokemon Model/Texture Data - 1.22 GB (0.99 GB in ORAS)
a/0/9/8 - 47.2 MB
a/0/9/9 - 48.0 MB
a/1/0/4 - Trainer Class Data - 7.33 KB
a/1/0/5 - Trainer Data - 19.0 KB
a/1/0/6 - Trainer Pokemon Data - 36.1 KB
a/1/5/5 - Static Encounter Data - 12.8 KB
a/2/7/7 a/2/7/8 a/2/7/9 a/2/8/0 - Battle Tree Data - 99.4 KB
Other Data
data/sound/ - Music - 417 MB (322 MB in ORAS)
CROs - 8.4 MB
code.bin - 5.44 MB
m/ - Title Video Data - 189 MB (41.2 MB in ORAS, under /a/1/5/2)
