Hi there - i too breastfeed all my kids, and like emma, you are on demand sometimes for what seems like ages in those first few weeks and my last baby was 10lb 9oz born, she was feeding for the UK i think in those first weeks and my god i felt like dolly parton on speed i can tell you, dont think my boobs will ever be the same again LOL, so in answer to your question i think for his size its not unheard of for him to want the amount he does, im assuming he settles after "the top up!" for a few hours??? if not, then maybe its better to have a word with your HV and see what she has to say as anymore at this stage would be abit much for his tum to handle.
All the best, hope its been helpful - kaz preg 17+1 with no.8 xx