My guess is that you have used your browser zoom function at some point on the VF Tab page,  and that your browser is remembering the zoom level for that particular page.  Make sure both pages are set at Zoom 100%  (Cntl - 0  Windows,  CMD - 0 Mac)  and then test again.  When I did this both types of tab were sized the same. 

There are some browser plugins that will set default Zoom and allow easy overriding of all open tabs,  but core browser function seems to be that its done page at a time. 

Weird. Must of been tired. I’m aware of zoom levels. Use it all the time. That was it.

Lay off the eggnog.