"Jumbo" is actually a reference to a circus elephant. Not just any elephant, one particular circus elephant. "Jumbo" was the name of a very large elephant, exhibited by P. T. Barnum in the late 1800s. He was an unusually large elephant and very famous. For a long time, his stuffed hide was kept at Tufts University in Medford, Massachusetts, but it was destroyed in a fire. In the United States, the three largest egg sizes are called "large," "very large," and "Jumbo" and that pattern is followed elsewhere. If something comes in a series of sizes, "Jumbo" can mean the larget size.