Less constriction in a smaller bore to produce the same pattern percentage as a larger bore, as mentioned above.
That being said, I'll add the following: From a table in Jack O'Connor's "Shotgun Book", it's quite clear that different manufacturers had very different ideas about how much constriction should equate to which choke designation (full, mod, etc). For example, in 12ga, Winchester used .016 constriction in its mod barrels, Remington .018, and Browning .024. (Generally speaking, if you measure chokes in Belgian Brownings, you'll find that they are fairly significantly on the tight side.)
