Why is my baby eating so much?

he is 6 weeks old and i feed him his normal 4 ounces and about 15 minutes later he starts acting like he is starving again. i try everything else i give him his pacifier play with him, cuddle, bounce him, put him in his swing, change his diaper, everything but he still steadily cries like he is hungry sucking on his fingers and trying to root into my chest. at first i thought he was going thru a growth spurt but it started 2 weeks ago and he is still wanting to eat like this. sometimes he'll keep eating up to 8 ounces in an hour and a half. he is a big baby but he isnt really chubby. i was just wondering if thats normal or is it something else im not thinking of... he also has been alot grumpier lately too. crying alot more than he used to. any ideas???