Use powers on bullets
If you compress the bullets but your powers keep their mass and velocity the same (= preservation of momentum), you haven't helped yourself a lot. The bullet will still hit you and will still punch a hole through you; just that the hole isn't a 9mm hole but a 1mm hole (or thinner). Actually, if you make a regular handgun bullet smaller, then the likelihood that it becomes a 'clean' through-and-through (instead of getting stuck in a bone) is a lot greater. The area of impact is smaller, so the penetration capability becomes greater. Kind of like, if you hit a piece of wood with a hammer, it might get some dents. But if you hit a nail with the hammer and the same amount of force, it gets driven into the wood very easily.
So, basically, compressing helps a bit with the size and severity of the wound, but doesn't eradicate the problem unless you go down to a micro- or nanometer level (so that the hole becomes so small that you don't realize it anymore). Also, the question remains what will happen with the bullet afterwards -- does the magic wear off, or is this a new way of creating super-dense materials?
If you expand the bullets, you've got more of a chance of 'stopping' it. Expand it from 9mm to 90cm, and it suddenly gets so much air resistance that it will drop to the ground within 2 or 3 meters, or get completely blown off-trajectory. Since it still weighs as much as a single bullet but has the size of a bean bag, you could compare it to getting shot at with a large piece of styrofoam. You might get a bruise, but nonlethal.
In both cases though, you need to be fast enough to work your magic! If there is any kind casting time involved -- the bullet will hit you before you've finished the spell. If you need to focus your attention on the bullet after it leaves the muzzle, you are SOL - human reaction time simply can't handle such speeds. The only way to actually stop bullets with that kind of magic would be creating compression / expansion fields where anything that enters the field becomes compressed / modified. And that brings a whole slew of different problems with it! Or you need 'program' your field magic to only work on projectiles above a certain speed, or something like that...
Use powers on space (and other intangible entities)
You could expand the space between yourself and the shooter. That way, it looks like you are 5 meters apart, but the bullet actually has to travel 5 miles to bridge the distance. Should it arrive at the target at all, it will have lost most of its momentum and be completely off-target.
Also, you could compress the space inside the pistol / gun's muzzle. This will definitely mess up the shooter's aim because the bullet leaves the gun much too early. The powder's explosion capabilities might not all become transferred to the bullet, so it's not that fast anymore.
You could compress the air between you and the bullet so that it becomes as dense as water and robs it of much of its velocity.
With compression/expansion, you automatically get awesome refridgeration / heating powers. Expanding a substance but keeping its mass the same cools the substance (the principle on which fridges work). On the other hand, compressing a substance radically heats it up (huge example: hydrogen clouds in space are slowly drawn together by gravity and become tighter and tighter until one day they're dense enough to ignite into a new star).
Depending on how long-lasting the effects of compression/expansion are, you also have a way of manufacturing super-dense materials that are an awesome bullet shield although they are super-thin. Doesn't do much for the weight (still weighs as much as a 6-inch slab of steel), but at least it looks nicer than a tank.
Such super-dense materials should also have some other interesting physical properties, depending on how you achieve the super-density. Either you decrease the space betwen individual atoms, or you shrink the atoms themselves, too -- any way, you will be messing around with some of the most basic forces in the universe, and as a result I bet you will be getting all new and exciting physics out of it!