How to reproduce:
1. Open user's attached project
2. In editor, go 'Assets > Build Asset Bundles'
3. Go to 'Project/Assets/Streaming Assets' folder
4. Notice file sizes

Actual behaviour: Asset bundle includes another bundle instead of a reference
Expected behaviour: Asset bundle should only contain the reference to another bundle

Reproduced with: 5.4.0b22, 5.4.0f1, 5.4.1f1. 5.4.3p1, 5.5.0f3, 5.6.0a5
Not reproduced with: 5.3.6p5, 5.3.7f1, 5.3.7p2, 5.4.0b21
Regression introduced in: 5.4.0b22

Dev Note: This issue requires a refactor of a core system, which is currently in-progress. The development team expects this refactor to be complete in version 2018.1. At that time, this issue will be automatically re-opened and re-tested to verify the fix.

Fixed in: 2018.2.0b1 using Scriptable Build Pipeline package.