Steps to reproduce:
1. Open “BoundsSize.zip” Unity project
2. Open “Main” scene
3. Select “Sphere” object in Hierarchy
4. Change object’s transform rotation to (45, 45, 0) in Inspector

Expected results:
Renderer.Bounds.Size does not change after changing rotation
Actual results:
Renderer.Bounds.Size increases significantly

Reproduced in: 2018.2.0a1, 2018.1.0b7, 2017.3.1p1, 2017.2.1p4, 2017.1.3p1, 2017.1.0a1

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By design:
The renderers bounds are an AABB of the meshes bounds rotated by the transform, this is for performance reasons as calculating the position of every vertex and creating the bounds every frame for every object is not optimal.