Bay scallops are among the smallest of the scallops, corresponding to 70/120 using the numerical scale described above — meaning that there would be between 70 and 120 meats per pound of scallops. Bay scallops are particularly sweet and delicate, but not well suited for pan searing.
At the opposite end of the size spectrum, sea scallops are the big boys — in the range of 10/40 per pound or even bigger (U/15 or U/10, for instance). Reaching 1½ to 2 inches in diameter, they can be pan seared much like a filet mignon - with high heat producing a crispy outer crust, while leaving the center tender and medium to medium-rare.
Most scallops are harvested by boats dragging chain nets across the ocean floor. Diver scallops are harvested by divers who jump into the water and collect them by hand. The term "diver" does not itself imply a size, but these divers generally pick the largest scallops they can find, so diver scallops tend to be in the 10/30 range.
Aficionados say diver scallops are more ecological because the divers only pick the bigger, more mature scallops, while leaving the younger ones, which allows the population to replenish; whereas dragging with chains is indiscriminate and sweeps up other shellfish besides just scallops