Montreal island has never been more populous than it is now. Population peaked in 1971 at 1.95 million in 1971 and then suffered a major decline as the economy flunked and reactionary/paranoid anglos foreseeing the end of the world packed up their bags split town. Montreal island has probably reached 2 million by now (just not officially yet) and the metro has now reached 4 million, again not officially though.
And yes dirt devil, the island is dense, but not as dense as it could or should be. The truth is, that outside of the central neighbourhoods (downtown, Sud-Ouest, Verdun, Plateau, Outremont, Westmount, Cote-des-Neiges, Villeray, NDG, HoMa and the rest) the island remains suburban, industrial and at some points even rural.
The island could easily welcome another 2 million within new mixed medium-density hoods no problem!