Steps to reproduce:

1. Open Unity on Mac Book Pro with Retina display;
2. Create new project;
3. Add Terrain gameobject (GameObject -> 3D Object -> Terrain);
4. Select any TerrainComponent tab, that has brush icons (for example, Raise/Lower Terrain);
5. Observe the brush icons.

Actual result:
Brush icons are very small (see attached actual.png screenshot).
Regression introduced somewhere in 5.4.0.

Expected result:
Brush icons should look consistently to other platforms (Windows) and to previous versions of the Editor (see attached expected.png screenshot).

Notes:
- Reproduces on OSX 10.11.1 with MBPro having Retina display;
- Reproduces: 5.4.0a5 (6b220773f9c9), 5.4.0a4 (c877ead45aff);
- Doesn't reproduce on 5.3.0f4 (2524e04062b4).