You're right. Strangely, LabelStyle is honored in terms of FontSize, -Slant, and -Color in Slider2D.
The same applies to Slider:
Manipulate[Style[x, Darker[Green], Bold, 14],
{{x, 0.5, "x-Wert"}, 0, 1, ControlType -> Slider,
Appearance -> "Labeled"},
LabelStyle -> {FontSize -> 12, FontColor -> Blue, FontSlant -> Italic}
]
To conclude: I think it's simply a bug in the Manipulator control where they forgot to take FontSize into account.