For reasons of consistency, tabs should not expand the length of the tab bar, since they do not do this in pretty much every KDE application that has a tabbed interface (e.g. kate, dolphin, ksysguard, okular, etc.)
They do in Okular and Kate, actually. This inconsistency is something that we need to address one way or another.
Oh, they do? Then it must be kvantum because they don't on my system. But I agree that we strive for consistency in this regard.