"Space Rock"...

First of all two cue balls have so little mass that any gravitational force is so negligible that you cannot even consider that to be a real example of this situation. The rules of physics which apply to everyday life on earth do not apply to the everyday life of super massive objects. Taking that into account, if you look at it in a mass comparison, the Earth today is roughly 2x as large and 10x as massive as mars, whereas the moon is 1/4 the size and 100x less mass than the earth. If you look at spacial displacement only it will end up making the situation seem far worse. Now, earth back then was actually roughly 90% it's current mass. When earth light and mars II smashed into each other the force of impact superheated the entire mass of both objects and ejected roughly 25% of their combined mass. Now the remaining 75% of the mass begins to be reformed into a spherical shape by the gravitational forces that exist. This is indeed a scientific law that when an object obtains enough mass the force of it's gravity pulls everything into a spherical shape. This is how the two objects meld. Now the remaining 25% of the mass is floating around this anorexic earth and is sorting out into 2 categories, one falls to the earth returning most of the lost mass back to our planet to form the earth that we know of today. The other remains in orbit and begins to form a heavy planetary ring. This ring of molten rock is too heavy to remain a dust ring like that of Saturn or Uranus, so the rocks begin to clump together. Slowly piece by piece they gain mass, absorbing other smaller pieces in their path to eventually meld into the moon. This theory is directly related to the theory on how the earth and every other large object in our universe, from stars to asteroids, originally formed.

So to answer your questions: 1. Yes, this theory uses different forces like gravity to explain the fusing of the two worlds. It also uses the mass not volume of mars to explain what happened. We don't know how large or dense this planet was but it is largely assumed it was much smaller than early earth. 2. Most of the mass remained with the earth. The remainder of it's mass and that of the earths makes up the moon of today.

Basically, we needed to explain why the moon is so much like the earth when every other planet with natural moons have a composition that varies greatly from moon to moon. You cannot use the leftover theory because then the moon would have different ratios of the same elements. But it does not so science had to devise a way for such a case to logically exist. And in order to logically force 10% of the earths current mass into permanent orbit it required the force of such a collision. To me the other theories on how the moon formed are far less logical than this one.