I have set android:textAllCaps=\"false\"
in my android.support.design.widget.TabLayout
thought it is showing the Tab Title in All caps only.
For those who can't get working other answers.
Defining a style is working fine when you have single line tab text. If you take a close look into the TabLayout, you'll see that it's using a field design_tab_text_size_2line when the tabs has more than one line.
The only way I could find to effect this field is to override it in your dimen file.
So put this in your values/dimens.xml
10sp
Hope it helps.