Why does it look as if the Battle-Armor is about half as tall as the Atlas in that one picture?
It's called perspective

And the Longinus isn't standing on the ground, it's in the air, look at the feet

What you see in the Inferno shot is an early explosion of the PPC. They are not balls like MW2. They are more like a hybrid MW3 and MW4..
Hmm...

If both a) are early PPC explosions, what did they hit?
Looks like they are blue sparkly balls flying through the air to me, like MW2 PPCs, that's why I mentioned it...
Highlander is the one who thought they were slow-moving

b) looks more like like a PPC explosion, but what do I know?
