Well, not really a theory but a tactic to use. If you are looking for bites the odds do increase with the smaller baits. There are still plenty of guys out there still throwing the big 8" and 9" swimbaits looking for that one big fish
Myself, I downsize this time of year just looking to get bit
If I catch a bass fine, but I am content with action from perch, calico, etc. Things are slowing down by the day