This conversation crops up occasionally - probably much more often than I chance to see it.
Projectiles DO get bigger, well, cannon projectiles, anyway, missiles remain the same size, to my knowledge, though I've honestly never looked too closely.
Beams remain the same size, indeed, but that is okay, I think, initially, I felt the same disappointment as I tried them for the first time but with a little bit of imagination you can do many more interesting things.
Cannons:
With cannons you can arrange the barrels of a weapon into a shape to provide the feel of a larger projectile, you can even have one solid-looking mass of powerful shots and put a string of single block Cannons behind it in order to give it a little trail, and give it a bit more of a pretty effect to it, like a plasma bolt.
Here is a rather basic example of many barrels arranged into a 'shape'
Beams:
Enormously powerful spinal mounted beam cannons are super sexy awesome. this is a FACT.
But, as you say, it looks unimpressive! or does it? by arranging the cannons into a shape you can make it look way manly, the other barrels don't even have to be effective, just one block for looks.
Proof of spinally mounted beam cannons being the sex:
By using a little ingenuity...
In the above cases, none of the beams are 'fake' they're both several hundred thousand blocks worth of planet-destroying awesome.
Now get out there and make some ultimate badass weapons!