Dragon Ball Landmark says his power and speed increase "several times", but a legitimate multiplier was denied because his Battle Power only increased from 24,000 to 30,000, which is only a difference of 1.25x (as opposed to 2x or even 3x).
Could be referring to his physical stats exclusively. In his normal form, Zarbon's opening move versus Vegeta was his Full Power Energy Wave, but after transforming, he only ever uses an energy attack as a decoy, near-exclusively overwhelming his opponent with his sheer size and brute strength and employing full-body charges and grapples. Granted, he also only fights a grand total of two times, but I digress.