You sure about that? I had several 5 gallon buckets on my last grow that were PACKED with roots all the way to the very bottom. I do think (based on what I've found and some opinions on here) that there is a point where the returns become diminishing on the larger pot sizes (depending on the auto strain,) but generally the bigger the pot, the more potential for a bigger plant since there's less chance for root contention/root binding.
The watering frequency would depend more on the qualities of your growing medium; not just the amount of growing medium. 5 gallons of Pro-Mix with a good deal of perlite is going to drain much faster than 5 gallons of dirt you pulled out of your back yard. Know what I mean?
Just some food for thought.