Please consider using the specified pipe size
Often fountain spouts are the last item specified on a project. During construction the required pipe sizes aren't always known. The installer must guess what pipe size to use for the spouts. This can lead to problems later on if the pipe is sized too small.
In general the larger sized spouts are not affected as much with changes in pipe size as the smaller ones are. The Large Karlo, for example, works fairly well with plumbing down to ½ in. when the water flow is under 3 GPM. At larger flow rates it doesn't work as well. All in all, that's pretty good.
The smaller spouts may not behave so well