I'd say it's two things:
1) Dallas has played awfully for most of this series.
2) Playing 5 guys 6'4"-6'8" will make you quicker... making it a heck of a lot easier to close out on guys on long shots. What the Warriors lack is any kind of interior defense. Their perimeter defense is, really, just fine, if not pretty good. Particularly against a team like the Mavericks, which plays so traditionally.