I believe the fare difference is more to do with the fares offered in the departing market than whether they're one way or return.

Fares sold in Canada, whether one way or return, are sold at a certain price point, with a certain level of availability. Fares sold in Europe, whether one way or return, are sold at a different price point with different levels of availability.
When searching a one way fare from Canada you'll get Canadian prices, when searching a one way fare from Europe you'll get pricing for the departure country. You'll note that return flights from Europe will not be any cheaper for the departing leg than if they are booked one way.

Why fares are different for different markets is based on what WestJet thinks they can sell them for there, plus exchange rate fluctuations if you're not paying in the local currency.

While people are finding fares from Europe to be more expensive than fares from Canada at the moment, I've often found it to be the other way around, and have booked my round trip travel as separate one ways to save money in the past. For example, while Business currently appears to be more expensive from Europe, Premium seems to be cheaper.