I think… this Mister want to buy 4 yukkuri for snack. Chen warned him that usually 2 is enough (to stuff you full).
But he doesn't let anyone telling him what to do, so he insist on four yukkuri. And add an Udonge just to punch back at Chen.
So here, he discovered that this place sell bigger yukkuri (those are probably grapefruit sized, compared to the glass) Now he can't risk Chen giving that "I told you" look, and have to finish all four, plus the Udonge.
But Saline said there's a pun involved, so not sure anymore.