The title of Rebecca's boobs' memoirs are "Large and in Charge" by Right Boob and "Slightly Smaller Than the Right" by Left Boob.
In the pilot of Bette Midler's short-lived sitcom Bette (2000), the Divine Miss M. tested the perkiness of her breasts by putting a pencil under them... and then a notepad, clipboard, and various other objects. Rachel Bloom was fascinated and discovered her bare cleavage could support a paperback novel, so she swiped the gag for her song "Heavy Boobs."
In the song "Heavy Boobs," Rebecca says her breasts would "be a catch at Fisherman's Wharf." At that moment, she intended to show a bunch of people with fishing poles catching breasts, but there was neither time nor money in the budget to include this sight gag.
During her youth, Rachel Bloom watched a breast reduction surgery on The Discovery Channel and was traumatized when she discovered the inner tissue was yellow fat. This served as one of the primary inspirations for the song "Heavy Boobs."