At first I thought the guy was giving her a "feel". But on second look, it appears that the hand grasping the large boob is the woman's hand. The sleeve is the color of her shirt. She is holding her own boob. It makes even less sense after that.
Discussion
Unless his hand is squeezing in between somehow
I think that's it. Looking more closely, it appears the woman's forearm descends lower and around the man's, reappearing underneath the hand. That is unless the man has six fingers on his right hand, in which case I know a man who is looking for him.