I guess I'll take a shot at this....
The "Larger bells for the fast focus" has me scratching my head.....unless you mean the ocular bell (the bell closest to your eye) rather than the objective bell (the bell closest to the object you look at).
I suspect, though, that you've labeled the objective bell correctly, and you ran into one of the all too frequent sales reps that think they can't say "I don't know" or "there is no good reason".
The purported reason for larger objective bells on new scopes is for better light gathering in low light conditions. Part of that may be due to newer scopes being higher powered, and in variables, wider magnification range. A 4-12X scope will need a larger objective for good light gathering at the high end than did the straight 4X we all used in Thee Olden Days.
Another factor in larger objective bells? Because they can.
Today manufacturers can grind (and coat) a larger lens cost effectively than they could 30 years ago- allowing scope designers to come up with those wide range variables you see today (like 4-16, 3-12, etc.)
Another reason for large objective bells? It sells scopes. Everyone knows more is better, right?
And having a bigger scope bell than your buddy is better, too, right?