I think it depends on your lo's age. My lo is 18wks and will start to play with the nipple with her mouth-move it around with her tongue, etc. I've heard some moms say their lo gets real squirmy. In general - let them have as much as they'll take is my philosophy, and I know I've read somewhere on these boards to keep an extra half ounce in the bottle more than they typically take so it is there if they want it, and when they consistently drain the bottle, add another extra half ounce - that way you can stay on top of growth spurts, or at least be a little more aware that.
That being said sometimes a bottle nipple flows faster than the boob, so maybe your lo is taking in more before his tummy can get the message to his brain that he is full? Breastfeeding never took off with us, so this is an area I don't have any first hand experience with, but maybe if you think his brain isn't getting the message that he's full - give him what you think would be a proper feed, wait a few minutes and offer more, and if he takes it you know he's still hungry and a "proper" feed is a little bigger? HTH