It's all personal preference. Looking around today, a lot of drummers use hugely diverse kits with all sorts of percussion sounds derived from them. Main reasons, (in my opinion, as a metal drummer and owner of a very large kit and a very practical sized kit) would be price, ease of travel, and depth of your sound.
Obviously, the large your kit is, the more initial capital investment you will have to make and the more it costs to maintain. My 10 drum 77 ludwig is expensive as hell every time I re-head.
A small kit travels more. If everyone is coming over to jam at your house, as I had the luxury of throughout gradeschool, then a large kit works. If you are constantly carrying it around, then obviously a small kit is the way.
Sound is huge as well. Personally, I find that in metal drumming with covers and originals, that having a plethora of cymbals to choose from is the best way to diversify your sound. Maintain a 3 piece kick, snare, tom combo, but on top of that buy a rack and get yourself some splashes, chinas, bells, heavy/light crashes, and an extra hi hat to lock when you run the doubles.
Buy cheap crap off of ebay to get a feel for the different applications of each cymbal. When you find one you like, price out some nicer brands and ALWAYS test them out before you buy in your local music shop. Personally, I like zildjian, meinl, and staag for my metal fun times.
Smooth sailing~