I think as a general rule of thumb a lighter vehicle is better. When you start out with a heavier vehicle it turns into a big weight gain cycle: Vehicle weighs more so it needs stronger (heavier) axles/suspension, now you need to bump the power to move a heavier vehicle, and then you need to upgrade the drive train to handle the power. Each of these adds weight.
Or you are going to place more stress on standard components if you increase the weight too much with all sorts of aftermarket gear: winch, offroad front bumper, hd skid plates, rear tire carrier and big rear tire, hd rockerguards, spare gas cans, roof racks, dual battery setup, OBA, etc. etc. etc. A fully stocked rig can add hundreds of pounds to a stock vehicle's weight. Oh, and toss in a couple passengers . . . there's another 300 - 400 lb. not to mention the driver.
One of the sweetest rock crawlers I've seen used a tube-frame chassis. It had a mid-engine air-cooled Porsche 914 powerplant. No need for a radiator and cooling system. Light weight magnesium crank case (compare this with cast iron chevy small block v8). I think the owner figured he shaved 700 lb. off the weight of a typical rock crawler with his setup. So he was probably weighing in at sub-2000lb. weights.