Interesting, Le Celticant, maybe GBX made them a bit smaller on purpose to help make them harder to hit.
Probably not, but it's a remote possibility.
Personally I find it scarier, since they can still jump on you and rape you to death. Spike seemed a bit smaller in A3, maybe it's just me, or the lack of dorsal tubes. I remember the runner in AvP2 was small, so was the warrior, but he stood up like a human and had a cool running animation.
I remember the runners were so damned small, and could do the old superman across open levels. Used to get so many kills as the runner. Mmm...