Well...without any details on the engine...and with you sounding a little inexperienced (no offense intended there, just that you're working with a shop to get it jetted instead of doing it yourself) but yet even with being inexperienced you still feel it's too big...that leads me to believe it must be running poorly or giving you some impression that it's too big, which means it likely IS too big.
Now...my own personal belief here, is that most of the time smaller is better as far as carbs go unless you're talking about a large H/C/I street/strip only car. Can they jet it down to "make it work"? That's debatable. IMO, they "can" but, no they can't. It'll never run as good as a smaller carb will. It's gonna sacrifice somewhere....either high or low RPM in order to make the fuel curve acceptable. They can jet it down and make it run, but it'll never have the throttle response and bottom end that a 600-650 will.