This question has been floating around alot lately, and seeing as I have nothing better to do, I just figured I'd make this short tut.
So, you want to know how to make your weapons huge and CARTOONISHLY oversized?? Here's how!
What you need:First, you need the one tool that no Halo hacker should be allowed to live without, Halo Map Tools 3.0 (find if yourself

).
You also need Milkshape, which you can find yourself also! GET OFF YOUR ASS NEWBS!!
Ok, first real step. Open up HMT 3.0 and your map along with it (I'll use Bloodgulch Xbox for this tuturial). Once your map is opened, pick a weapon and export all or most of its LOD's (or Model Types for you big newbs; for this tutorial I'm using the flamethrower):

Once you've exported all of the models you feel confortable with, open up Milkshape and import your models in whichever order you see fit:

Now select all of your vertices (they will change to red) and look over at your toolbar. Select 'SCALE' from the top section of the tools:

Go down to the blanks below and in the blanks above the X, Y, and Z coordinates, put 4.0 into the blanks, then click SCALE.

If done properly, your model should have quadrupled in size! Export your model, inject it back into HMT, save your map and your done!
ENJOY YOUR NEW HUGE ASS FRIKK'N WEAPONS BABY!!

Edit: Am I the first person to make a tut on this? WEEHOO!

Enjoy everyone.