Question: How can I make my 8 year old nephew understand that he is too big for me to pick up and give a piggy back? 

Answer: tell him "no", and offer to do something else. no piggy back rides though. let him know it hurts you, and be firm while suggesting other things to do. another approach might be "eight year olds are big boys- they don't need piggy back rides. want to play checkers?" or some such.
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