It's really common for fairly young babies to have OS nappies on the larger sizes and parents often worry that the nappies won't last, but remember your LO is hardly moving at the moment. Once babies start rolling, crawling, walking etc they often slim down quite a lot. My LO never went down the rise settings again but we were able to fasten the waist and legs tighter once she started walking.
With the Ittis, remember that Ittis are low rise nappies so you shouldn't expect them to have the same kind of fit as a bumGenius or similar. That said, if the way you're using them is working, there's no need to change it! As LO grows, you will just find the nappies get lower rise (but if LO has a chubby bum, as mine did, you may find the rise stays looking the same as there's less bum in the nappy once they slim down so the nappy can stay the same height around the waist despite LO being longer!).
Looking at the photo, your LO has good chunky legs so that is probably why you get away with the rise on the bigger settings. You have the waist quite small so I wouldn't worry about room to grow. The rise will get lower (think hips rather than waist for Ittis) and the waist has plenty of room to expand. If you were to go for a more standard Itti fit, you'd take the rise setting down one or two settings so it's low, but then you'd have the waist looser as it would be around hips rather than waist and you'd need the extra room for his thighs.
