Apples/Oranges.
Of course that 12 ga seems clunkier than the 16 ga. Look at the difference in barrel length. Take a hacksaw and knock several inches off the 12, cut it down to the same length as the 16, and It'll swing a lot quicker, maybe almost as easy as the 16, but it'll still hit like a 12, and the 16 will still hit like a 16.
The bottom line is, if you want to shoot a gun that hits like a 12, you have to shoot a 12...or a 10.
If you're happy with a gun that hits like a 20, then why fool around trying to find ammunition for a 16? Shoot a 20. M12, if you like.
The M12 is a great classic gun, well designed and built, no question. You still see some die-hard trapshooters putting up good scores with their pre-1964 12 ga M12s. In fact I watched an old codger, (like me
put all the youngsters in that shade just last week with his M12, and that gun must have a half million shells through it by now.....
BUT you'll
never, ever see them shooting a 16, although it's perfectly legal, and can shoot the same 1-1/8 oz. load.
The 16 is a curiosity, almost like the 14 ga. There was a time, 60 years ago, when it might have hit almost like a 12, but today it deserves a place of honor, hanging over the fireplace.
If you want a really quick handling, lightweight gun, shoot one of those 5.5# Spanish doubles like an AYA.