Had a little free time today so I broke out the 5906, the Beretta 92FS and the dial calipers. Both the 5906 and the 92FS have their factory grips installed. The 5906 is wearing the factory curved grips, so it looked like if I was using the S&W straight grips the front to back dimension may be larger especially at the bottom. I had a S&W straight grip on the work bench but didn't install them, since the basic 5906 came through with the curved grips that is what I used. The 92FS is wearing the factory plastic grips and is current production with the radiused grip frame.
Stock grip to stock grip. Here is what I found.
"Measurements are approximate"
SMITH & WESSON 5906:
Width (side to side): 1.250" - 1.260"
Depth, Front to back measurement, starting from the bottom.
Base of Grip Before the Flare: 2.130"
Middle of Grip: 2.100"
Top of Grip: 1.940"
Beretta Model 92FS (current production with the radiused frame grip cut)
Width (side to side): 1.370" - 1.390"
Depth, Front to back measurement, starting from the bottom.
Base of Grip Before the Flare: 2.220"
Middle of Grip: 2.105"
Top of grip: 1.970"
You can see the 92FS is bigger in every dimension. If I had an earlier 92FS with the non radiused grip frame, the front to back dimension at the top of the grip would be even larger. Coincidentally the magazine dimensions are so close they will insert in each other's magazine well. Handling both at the same time, I found the 5906 smaller and slimmer than the 92FS.
Now the OP had originally asked about comparing the 5906 with Hogue rubber grips and the Beretta factory grips. He may be onto something as I bet they would be very close in width.