They're not "comparable" at all. You're probably comparing the lightest model 3 (which you cannot buy anymore, the newer models are heavier) to the heaviest Audi A3 model on the market otherwise they aren't "very comparable"
Same for the model S, it's on average about 200kg heavier than a BMW 5 series across the range from most basic model to heaviest on both.
If a car has a 500kg battery, and tech improvements can make that 200-300kg for the same range that would be quite good. Handling improves, safety improves, the suspension can be simpler and in the end also range in stop&go traffic would improve with less weight.
Same for the model S, it's on average about 200kg heavier than a BMW 5 series across the range from most basic model to heaviest on both.
If a car has a 500kg battery, and tech improvements can make that 200-300kg for the same range that would be quite good. Handling improves, safety improves, the suspension can be simpler and in the end also range in stop&go traffic would improve with less weight.