The tabs layout algorithm in Tabs Studio tries to minimize number of rows, distribute tabs evenly between rows and minimize tab movements between rows as you add or remove tabs. The last goal to minimize tab movements between rows sometimes creates quite wide tabs:

Extra wide tabs in the top row
For the new release I’ve reduced the priority of no tab movement and for the same tabs the new look is this:

More priority for shorter tabs in the new release
Plus I’ve added the new ShortTabsPriority property that you can customize from a custom style. It is a double value. Value of 1 is old behavior. Default in the new release is 3. Value of 10 is a high priority and 100 is a very high priority. Following is an example that you can use to customize the tabs width/movement balance:
<Style TargetType="TabsStudio:Tabs" BasedOn="{StaticResource DefaultTabsStyle}">
<Setter Property="ShortTabsPriority" Value="10"/>
</Style>

High priority of short tabs
Download link: Tabs Studio v2.7.5.