Thin...
I wouldn't say they sound "thin". I would comment that the mids seem kind hot. Enough to maybe overpower the lows a bit. Try cutting your midrange a couple decs. See if you like it. Low end sounds tight, and not over emphasized. you could add little sub if you wanted too. Naw, It's not too thin.
I listened to Neck Breaker, the Dark chant, Caught in the Belly of the Whale, Cipher, Extreme Pens, and Warning.
It's all good. Just a little hot in the mids. What do you monitor with when you mix? You might want to cut a CD or tape and give it a listen on other systems to see how it translates from system to system.
Nice work!
-L