You would have to reach "into" the tab strip component and provide your own widths to the component; it doesn't automatically scale its inner components to fill in the way you're expecting.
There's a hack of sorts to accomplish this via the tab attributes dataset documented here:
Or this thread has a different, more "imperative" approach involving scripting to directly modify the inner components. The script in this post is modifying the overflow arrow panels, but it could be modified to look for the inner com.inductiveautomation.factorypmi.application.components.tabstrip.TabComponent instead pretty easily.