By design:
Yeah looks like it's happening for non-TextMeshes too (e.g. legacy GUIText, 4.6 UI) have the same problem too - there's some "space" around the letters (font? line size? kerning? etc.). So the letters are never flush up to the corner.

Steps to repro:
1. Download the attached project (484834_small_repro.zip)
2. Open the project and open the "TextMesh" scene
3. Select the "TextMesh" object and center the camera on it
4. In the Inspector change the "Font Size" property
5. Notice that the text is being rendered in different position

Expected results:
Changing font size doesn't change the position where the text mesh is rendered. i.e. the top left corner should always be at the same place when changing the font size.