As we walked, we came to realize that many of the geese were noticeably smaller than the Canada geese we usually see--some seemed not much larger than the mallards, in fact--though otherwise indistinguishable from the other geese. There are several subspecies of Canada goose, varying considerably in size and length of neck and bill. The Cornell Ornithology Lab's All About Birds site notes:
At least 11 subspecies of Canada Goose have been recognized, although only a couple are distinctive. In general, the geese get smaller as you move northward, and darker as you go westward. The four smallest forms are now considered a different species: the Cackling Goose.David Allan Sibley offers a further guide to distinguishing the various subspecies here. The Giant subspecies is the kind we usually see, having become common in recent years after having once been thought extinct, and is the largest goose in the world, sometimes weighing more than 20 pounds.
Below, in a cropped version of part of the photo shown above, you can see a pretty clear difference between the longer-necked birds at the left and center rear and the smaller-bodied, shorter-necked birds elsewhere in the scene. I don't know which subspecies were in this flock, but it was fascinating to see so many smaller geese, whichever kind they may have been. My resident bird expert says he has not ever been aware of having seen these smaller subspecies before in his 15 years of birdwatching in Minnesota, so although they were "only" Canada geese, this was rather an exciting "spot."
Note, 11/10/2008: I have replaced the original photo below with one that I've marked to more easily point out the contrast between examples of the larger (in blue) and lesser (in red) subspecies.