As O2bnme described, the faster you go, the more lift you develop for any fin size, and the iS tuning is more about top end speed, so typically the fin tuning remains a little bit smaller and the lift (fin performance) kicks in with extra board speed.
The stock sizes as provided (36 and 44cm for iS125) are selected for their maximum range, covering probably the middle "80%" of the iS125's range (typically..)
If you are underpowered on 9m (marginal conditions) then adding more fin ~48cm will help, (planing slow is always faster than not planing
) however as you come more into the power band the extra fin starts to detract from the top end potential of the iS, and lighter riders may even find the excessive lfin lift counterproductive to the overall trim of the iS (if the fin is too overpowering, it tends to disturb the rider's loading/drive down thru the mastfoot, which is a vital ingredient in making the iS really fly straight and true across chop)
Depending on your trim/tune, you may find the 44cm is better for powered/overpowered 9m conditions, esp with 72kg rider.
Of course, with 9m (c/w say 7m) the difference in comfortable to overpowered occurs across a smaller wind range, especially with lighter riders on smaller (c/w FW) boards like the iS125. In this case, the fin change down (to 44 if you are on 48) can help - and may avoid a sail change down, at least for another couple of kts of wind increase.
Cheers ~ Ian