I can't give you much information about Paris and its population.

But you mention Constantinople as a comparison European city in the Middle Ages, and I want to point out that using it as a benchmark during this period is tricky. By the 15th century, Constantinople was at its nadir, having lost much of its vast eastern Mediterranean empire over the course of centuries.

Constantinople probably had a population of at least 500,000 at its height under Justinian in late antiquity. But the Plague of Justinian devastated the city's population. The empire was further depopulated by the disastrous Byzantine–Sasanian War of 602–628, and the Arab Conquest of much of the Byzantine Empire in the 630s.

The city's population recovered during the Byzantine Macedonian Renaissance in the 9th to 11th centuries, probably reaching least 300,000 by the 12th century. This easily made it the largest city in Europe.

But by 1453, when the Turks made their final successful attack on the city, it may have been down to as little as 50,000 residents.

So by the period in question, numerous European cities were larger, and it's not really saying much that Paris was larger than Constantinople.

Paris was only the largest city in Europe starting in about the 1300s which is at the tail end of the Middle Ages. For the other 90% of the Middle Ages, the city of Constantinople was the largest, richest and most advanced city in Europe. In 555 the city had a population of about 500 thousand, this number fell after the plague of Justinian, which was the first instance of the Black Death but recovered to approximately 500 thousand again in the year 1000. In 1204 the city was brutally sacked and occupied by invading Latin Catholic crusaders en route to Egypt. It never fully recovered and much of its wealth and Holy relics were looted or sold to Western European kingdoms to fund the economically unstable “Latin empire” that was in possession of the city from 1204 till 1261. When the Romans retook the city in 1261 it recovered to about 100 thousand but was hit again by the Black Death or Bubonic plague for a second time in the 1300s. The City fell into ruin and when the Sultan Mehmed captured the city in 1453 it barely held 50 thousand inhabitants living amongst the ruins of what was once the most opulent city in Europe. Only a few buildings from the glorious city survived, most famous among them is the Church of The Hagia Sophia or the Church of the Holy Wisdom which now functions as a mosque.