There seems to be no way currently to change the tab bar height. Thus it remains big even when the font size used inside is small. There should be a way to resize it. IMHO a hidden pref would be relevant. (Or maybe it could shrink to accomodate the font size.) *PLEASE NOTE*: this request is similar to bug 4398. But that bug applied to 0.2, plus it was marked wontfix after a comment totally irrelevant to the description (that comment refers to close buttons, which are not mentioned whatsoever in the RFE!). So please don't mark this bug as a duplicate (or reopen the other one, but it may look awkward).
For gtk3, this could be done via CSS: https://www.reddit.com/r/gnome/comments/4e9s4d/size_of_tabs_in_gnometerminal/