It appears the two abilities are separate from each other. I see Regeneration 5 (acid or fire) is listed in Defense and Reactive Regeneration is its own ability which specifies that the creature regenerates 5 hit points whenever it takes slashing or piercing damage, which could occur multiple times a round. Regeneration is typically just once per round.
While I can thematically see the view for having it stop growing if its not regenerating... the descriptions don't really match its growth with its Regeneration. I mean... it's otherwise always constantly regenerating so it would probably be constantly growing (which it doesn't, it tends to shrink, rather quickly). Only when the Reactive Regeneration is triggered does it gain Growth points.
I would have to say that even using fire or acid to suppress its normal Regeneration, a party still should avoid trying to slash and pierce it, because each hit will heal it and enlarge it. Unless the author or writer had some inscrutable idea or concept that didn't get translated or survive final edit, I would say that the two are separate abilities; one is suppressed by a specific attack, the other is triggered by a specific attack.
