The reason most projectile weaponry uses dense and relatively large projectiles is that a lighter object needs exponentially larger amounts of energy for similar amount of momentum. I.e. a 1-pound object needs 100 times as much energy as a 10-pound one for the same momentum.
That's just...
wrong.
Momentum, basically, IS energy, expressed in coherent motion.
All else being equal, a 1kg object imparted with the same energy as a 10kg object will travel at 10 times the velocity, and both will possess the same momentum
A common unit of measurement for momentum being kg*m/s, or mass times velocity, velocity being a product of acceleration over time, and acceleration being a product of force divided by mass, if (all else being equal) the force applied to the two differently massed objects is equal, so too will be their momentum, but not their velocities.
Back on topic, though, we really have very little to go on as to what Outsiders are actually resistant, or
vulnerable, to. All the answers, here, to that effect are largely speculative, and representative only of the opinions of those giving them.
On that note, it is
my opinion, that outsider resistance to magic, applying primarily but not solely to mortal magic, should be reduced in concert with the 'alien-ness' of the magic used against them, or of its source. The more '
not human' the magic, or its source, is, the less an outsider should resist it.