I have always something to say
According to this issue here, this behaviour is by design and is not going to be changed as the issue is flagged as "Won't fix".
However, you should be able to override the TabHeight via a Theme or even a Skinable Custom Control. I tested both in 6.4.0RC0a and the height of the Tab is sucessfully changed but this still is not fully working yet because of this issue here, which is scheduled to be fixed in version 7.0. What happens in this case is that the height of the Tab changes, the text is fully shown (I set the height to 40), but the images don't grow with the height increase... that's what the second issue will be fixing.
You can still make this work with some additional work on your behalf, by for instance creating Skinable TabControl for a few standard sizes and update the images in the control's resources to fit the height values you need.
Hope this helps,