Coming from NYC, and not having been to Montreal in ages, I can only suppose that you're on to something...
And yet, three things make me believe that there may be some truth to it.

First... is that NYC bagels have been getting bigger and bigger, to the point of becoming round loaves of bread. More is not better!

Second... is the relatively huge Jewish population in Montreal, with nearly identical roots to the NY one.

Third... they have nearly identical water supplies...

Worth a Mileage run to find out, eh?