Use two grates, as the others suggest, laying perpendicular to each other. It's just part of lump, though some brands are better than others (I use Royal Oak exclusively when it comes to lump). But really, it's just the nature of the beast. I kind of like it, it's more like a "real" fire, and every one is a little different! And having a mix of biggish and smallish is good, because the smallish will get more intensely hot, but the biggish will burn longer.
One thing I've noticed though, the bottom 1/3 of a bag tends to be almost all smallish pieces. I reckon that's the way they settle in the bag.