The 1st panel on the right side makes reference to official art drawn by Shimada Fumikane that was released for Episode 3 of Das Finale as seen on twitter #1370300292491661313.

The funny thing is that Shinobu is supposed to be even flatter than Isobe, but it seems that when girls are depicted as having breast envy, it's a common schtick that they tend to lose sight of things.