You must have a different 10ga than anything I've ever shot, cause I'm a pretty big guy and immediately notice the difference between a 10 and 12ga. Here's a simple table by Chuck Hawks, which he compiled from various reports by other experts in the industry.
http://www.chuckhawks.com/shotgun_recoil_table.htm
Note that a 3.5" turkey load from a 10ga imparts ~63lbs of recoil force. Let's note that's roughly equivalent to the recoil measured from a .450 rifle load - used to harvest African big game. Personally, I'm not going to subject myself to the force of elephant loads to take a 20lb turkey, no thank you...
One can argue that a 3.5" load out of 12ga typically kicks out ~50lbs, but that 10-15lb increase in recoil energy, which is coupled with a heavier gun traveling at higher velocity, is more than enough to notice.
With all that said - I've never had the chance to shoot a 10ga semiauto like the Ithaca mag, and we all know that semiautos redirect much of that recoil force (especially the gas models). Is that 10ga semiauto comparable with an Ithaca 12ga pump?