I'va also handled both the 226 (also 229) and 320.
I'd be hard pressed to easily compare the guns since they're so different on how they handle.
That being said, like other stated, I think that the medium grip is close to the bulkiness of the 226 grips, the medium 320 grip i feel is a tad smaller in circumference. .
You can just purchase a small and/or large grip for 35$ though
Top is a 320 FS medium grip, 229 has hogue grips (i dont have a pic with the normal ones), bottom is 320 compact/medium grip. Im looking for my comparison shot for the 226/9 with normal grips and the various 320's i have.
226 with newer grips
Edit: Don't forget about the possibility of installing an x-frame grip also. If you can handle a 320 x5 in a store and like it, order the FS grip from brownells, it's under 100$, comes with the magwell adapter and weight.
This is an x-carry 'medium' grip
Come to think of it, the x-grip might be close in profile/size than the normal 320 medium one... mmm
I don't know where you are, but if you want to try out a nice variety of grips for the 320 platform, I can ship a couple... can't be more than 20$ roundtrip for mailing the stuff if you want to pay for shipping...
More grips (compact large with a compact magazine, compact medium with a full size magazine and x-grip, subcompact):
Compact medium with the full size mag and x-grip (I really like how this feels in hand):