Why is Vancouver's International Airport busier than Montreal's, even though Montreal is bigger than Vancouver?

Vancouver is the Canadian landing point from Asia.

Vancouver is the main connecting point for flights to Asia in Canada.

Montreal is fairly close to other major international airports, including Ottawa, and Toronto. Vancouver isn't. The nearest major international airport to Vancouver is Calgary - a pretty fair distance.

Tourism

Vancouver is Canada's western flight hub.

Montreal is not Canada eastern hub. Toronto handles most of that.

Vancouver is a main west coast hub, Montreal is not a main hub.

Vancouver International (YVR) is a main connection point between Asia/Pacific destinations and Canada, so it handles a lot of flights and passengers because of it. It's the main western hub for Air Canada. Montreal's Trudeau/Dorval (YUL) mainly handles Montreal-area passengers and is a lesser hub for connecting flights as most Eastern connections are done at the much larger Toronto/Pearson (YYZ).

Has something to do with the big Ocean next to Vancouver.

Of course you know Montreal has two Airports and little Vancouver has only three of them.