In all cases where you have two mismatched tires on opposite sides of the same end you’re causing extra wear of the differential. In FWD cars the differential is contained in the transaxle. The differential contains a gear setup that allows the opposing wheels to turn at varying different rates while still both transmitting power,
In normal driving the wear on the gears that allow this is minimized. Using the example below, when you’re traveling straight the engine turns the ring gear, which spins the differential case. The case causes the differential pinion gears to orbit the axle’s axis, pulling the differential side gears and their attached axles along. When you turn, the differential pinion gears and the side gears turn relative to one another, allowing a difference in axle speeds.
When you have two different sized wheels on the axles, the side gears and differential pinion gears have to spin constantly against one another, adding a lot of unnecessary wear. This can cause premature failure.
If you must maintain two different tires on a FWD car, you face a tradeoff. You’ll reduce wear on the differential by putting them on the rear, but you’ll also make the rear more prone to losing traction in bad weather in a turn, like an entrance or exit ramp.