Meshes
High-poly? Low-poly? No problem!
Like textures, meshes also play a significant part in determining VRAM usage. They can have the same (if not, greater) impact on an avatar's performance.
Polygon Count
Ever hear how some VRChat avatars have more polygons than the entirely of some retro 3D games?
Optimizing polygon count, even for worlds, is a crucial step to making your avatar run well across multiple platforms. It may not be approachable for most, but I'll simplify some common approaches most 3D artists use when optimizing their models.
Polytool
Polytool is a paid Unity addon by Markoragon to simplify the process of optimizing a model. It can easily cover complex optimization steps like decimation, atlassing, and bone count.
Tuxedo
Tuxedo is a tool that can assist with optimizing your mesh from within Blender. It's completely free, but requires Blender experience. It can decimate for you, too!
Manual Decimation
If you prefer to have more direct control over what polygons are removed, you can do this manually in Blender.