[toni-a]

As far as I know, most pro zooms in Canon land had 77mm filter which is practical since you will be buying a filter size that fits all your lenses, this is especially true for the trilogy 16-35 24-70 70-200, when 17-55 and 24-105 were added naturally 77mm was chosen since many had already the filters. The new 16-35 is an exception but aside polarizer very few use filters, and even a polarizer isn't practical on a ultrawide.

So yes 77mm was chosen for practical reason just like 58mm for entry level zooms