My ds was 10lb 10 at birth and was on bottles from birth - despite him being so big he doesn't exactly knock back his milk. On average he takes about 4.5 ounces at 5 or 6 feeds a day but each feed can take up to an hour. He's on a size 2 teat so don't think a bigger one would make any difference - do you think some babies just take their time and don't necessarily need a lot of milk if they are big? He's now 13lb 10oz so has been putting on weight.
Another thing playing at the back of my mind is that with my first dd she was the complete opposite of my ds and she was a nightmare feeder and was diagnosed with a UTI at 8 weeks which when investigated showed that she had reflux in her left kidney. I guess I keep thinking that even though he seems to feed ok they told us that there was still a 20% risk that he could have the same problem.