Plane Change

I'm traveling out of Toronto to Fort Lauderdale roundtrip in November. The planes that originally booked on were 737-700's. Never got an email or notification that the flights have changed, but now they are on Max 8 aircraft's. Why would WestJet change this to a larger aircraft when the original aircraft they were going to use was empty, literally maybe 10 seats were sold or assigned? I have no problem, because the original seats I booked were in the 7th row and now my family got bumped up to EconoFlex More Space seats, so it's considered a win win for me. Just thought it being weird they change the planes like that.