Thinner wheels means thinner tyres and the combination of wheel/tyre weight reduction results in less unsprung weight (wheels/tyres/brakes/lower suspension) which is desirable to reduce the overall vehicle weight. It also means less drag when moving because there’s less rubber touching the ground. However, you need a calculated rubber area in contact with the road at all times to cope with the shifting weight of the car during acceleration, braking and cornering to keep it on the road as much as possible. This calculated rubber area determines the amount of rubber (tyre width) needed. Wheel sizes are usually calculated against the gearbox spin-ratios** to give an optimised performance: smaller wheels accelerate faster but need higher gearbox spins (or engine revs) to keep them going. Larger wheels accelerate slower but need less gearbox and engine spinning at the same speed as a car with smaller wheels.
** It’s all calculated from the vehicle max weight, gearbox and engine performance for optimal speed, safety and consumption. Because there is no engine and gearbox in a BEV, there’s no need for gearbox/engine optimisation so I suspect the calculation for wheel sizes are done on the weight and speed performance and also the visual aspect. Larger wheels which fill the arches simply look more expensive.