For art and drafting pencil brands that come in a wide range of grades, the softest leads have larger diameters than the others. This is so that they make heavy lines and don’t wear down too fast. The wood around these leads is quite thin. Nowadays this is not a big problem for manufacturing, but in times past, 6B (and softer) pencils were made thicker than the rest —
Here are some examples:
These pencils are about 20-25% wider than normal.
The thicker pencils required larger boxes —
A very old example, with hexagonal leads: