Yes, as NATO gets larger it becomes less capable of action. The more countries that join NATO, the more interests that must be balanced in order to come to a consensus for action. There are more and more countries that have different backgrounds because they are from different parts of the world. This makes NATO's mission more complicated.
While over extension can be taxing in many situations, be it financially, militarily, or enlarging a system, it is by no means an irrefutable fact that it will always be crippling. Accepting Georgia is accepting that country's resources as well as manpower, and no doubt their ruling body will contribute and cooperate with NATO as well as creating strong ties. So in a way it will not be simply be defenseless or a burden or a political problem. As long as NATO is organized which it should be, and respect is established, there should not be a problem with accepting Georgia.