Big Head

We went to brunch with an old grad school friend last weekend. We were having a nice time and Nick was being his usual prince self (which is the act he has at school and in public, but which falls away the moment he's alone with me and Chris). When all of a sudden he pointed to the table behind me and said "Look at that big head!". Chris wrestled his pointing finger down and tried to occupy him with something else. I didn't want to turn around and see what was going on, so I trusted Chris. Once we all got outside the cafe, Becky said "My god, did you see the size of that head?!" Chris described it as a sack of potatoes. How to explain to a little one that you can't make fun of big heads in hearing distance, but it's totally fine to do so when you get outside and out of earshot?