It is my understanding that the Emperor called the council.
Yes, but “under Constantine” sounds like Constantine ruled over it, which is a slander commonly made by restorationists (who then use Constantine’s ambiguous faith to assert that post-Nicene Christianity is pagan).
It was the Bishop of Rome whose authority was granted to Nicea, and that is no small point to be brushed aside as you have: many popes were martyred because the emperors coveted that authority and could not wield it; your careless statement gives them the authority that they martyred countless souls for.