Wim van Gruisen
Make it be mineral deposits beneath the earth. That explains the large areas, obviously a large vein of gold is going to be much more useful for a mining operation than a small patch of gold. So basically, make it so you're building your claim to mining, the player who builds the better, more profitable patches will obviously win.
Nice one. It also explains why you want to make your structure compact; long distances to the center means higher costs.
The only thing against it is that players place the tiles. That sounds more like planning than like discovering veins. Perhaps I can take a sexy theme like farmers deciding where to grow different crops, or algae farms growing different kinds of algae.