Hives develop over time.
Think of how large and dense cities like Shanghai or Tokyo are. Then imagine how much larger they could grow over the centuries, then the millennia if their populations kept growing. We've gone from 2 billion people in 1927 to 7 billion now. That's 1 century. Many worlds in the Imperium have been settled for thousands of years. You don't plan or plant Hive cities, they just develop over time as the population keeps growing.
Armageddon is an industrial world. Well situated and productive its population kept expanding as its industrial output grew. As industry and the Imperium grew ever more ravenous more and more land is given over to mining, or polluted or left barren. Populations cluster around the centres of production and industry and contract as livable space decreases. With massive populations densely clustered hives form as you build up and down to maximize space. This continues for centuries, then millennia and suddenly you have a Hive World.
In terms of materials Hives use everything other worlds do, rockcrete, plasteel, admantium, stone. Just more of it.
Last edited by Rems; 09-08-14 at 12:30 PM.