The 620 was meant to replace the 66 as a service gun. It is a stainless .357 like the 66 but is a 7-shot L frame with the modern assembly methods (2-piece barrel, MIM parts, etc.). I would guess that it is slightly larger and heavier as a result.
Specifically, the 620 is about 3 ounces heavier than the model 66 and is one of the most accurate revolvers ever produced by S&W because of the tensioned 2 piece barrel. Gunlbast published a review of the 620 and it grouped 9/16 inch at 25 yards with one loading and was under 1.5 inches with most. Personally on two seperate occasions I've managed to shoot 3 shot cloverleafs in single action from a bench rest at 40 feet that could be covered by a dime. The first time I was stunned, the second time I did it on purpose and was still stunned. However, the second time I was more stunned by actually being able to hold the gun steady enough and repeat on the sighting to group that well.
BTW, it will also fit most holsters produced for a 4 inch K frame because the L frame is just a smidgen larger than the K frame.