Maybe there's a sweet-spot amount of air pressure that belongs in the bladder to keep the expansion/contraction to a minimum? Who knows.
By the way, here's the best zipper-pull-tab replacement unit I've come up with, after several tries: Zipper Mend Pull Tab Replacement Quick Click | eBay
This one looks better but the metal fatigued and broke right away: Zipper Fixer Repair Pull Tab 1pc One Large | eBay
Haven't tried these yet: Zipper Tab Pull Replacements 2pc Lot of 2 New | eBay
The key thing to know is, on the YKK safety-type zippers on PFD's, there is a mechanism inside the slider that keeps it from moving up and down on its own unless the pull-tab is being pulled up or down. It's a safety feature so the zipper won't open on its own. Thus, the tab needs be a good fit and be adequately strong, etc. Key rings work, but are awkward.