I have set a style in App.xaml
<Style TargetType="TextBlock">
<Setter Property="Margin" Value="3"/>
</Style>
I realized that this also increased the headers in my TabControl which was not my intention.
I tried to restore hight in TabControl Header without success. (And tried the same with TabItem also)
<TabControl.Resources>
<Style TargetType="TextBlock">
<Setter Property="Margin" Value="0"/>
</Style>
</TabControl.Resources>
Update 1: one suggestion was to define an empty style for TextBlocks in TabControl. It did not help.
<Style TargetType="TabControl">
<Style.Resources>
<Style TargetType="TextBlock" />
</Style.Resources>
</Style>