When creating a tab control with the default width setting of "Label Width", the distance to contents (padding) is set by default to 8 pts.
If I change the padding of the tab controls, instead of reducing the tab width, it expands the size of the black box around the tab label. See before and after screenshots below:
This occurs both when I change the width immediately after creating the tab control (as in the above screenshots, where all tabs are still selected) and when I insert new headers, deselect, etc. Exiting/saving the layout has no impact on this behaviour.
Using the width setting "Label Width + Margin Of:" and setting the margin to 0 reduces the padding slightly but the padding from the black box to the text remains so there is still some space.