ATM has a payload of 48 bytes and a 5 byte header. IP, because it is variable size, you have to look at representative cases. Small payload carries a larger penalty - on the order of about 37 percent. Internet telephony is an example of a case in which there is a small payload. With delay sensitive communications there is typically a smaller payload.
File transfer has a larger average payload.
In packet versus switching, there is a trade-off between the efficiency of overhead versus bandwidth efficiency. Circuit switching is low overhead, but dedicates a fixed segment of bandwidth to the communications session. In packet switching all traffic is statistically multiplexed across a shared pipe. The price is latency uncertainty. Packet protocols are great for multimedia.