It’s about the larger base. Mogul (screw) base light bulbs or mogul base
Mogul Base Bulbs
bulbs have larger screw bases (E39) than the standard, medium (E26) screw base bulbs. In fact, the “39” of E39 and “26” of E26 each refer to the millimeter width of the respective screw bases (i.e., E39 base bulbs [...]