Re: "So someone decided use different battery size for Model 3 "
"Unfortunately, nobody is doing a reasonably priced car half the Mitsubishi size"
Welcome to the wonderful world of engineering (that isn't meant to be sarcasm, just an observation).
The problem with cars is, basically, frontal area. As the arms race to make them bigger continues, sports cars of the MX5 variety increasingly can't see where they are going in heavy traffic. There's a race upwards as well as sideways.
But frontal area determines drag and has a significant impact on fuel consumption.
Then there's safety. A small car has less room for crumple zones so it has to be made stronger to protect the occupants - hence the immensely strong chassis of the original Smart.
The result is that the fuel consumption of small cars is little better than that of large ones unless major compromises are made.
Batteries take up a lot of room, especially if properly cooled, and add a lot of weight. The tl;dr is that the bigger the vehicle the easier it is to make it electric or PHEV, especially when you consider that the engine of the PHEV is going to be roughly the same size whether you try to shoehorn it into a Corsa of leave lots of room round it in a Mitsubishi.
The result is that the Yaris Hybrid, for instance, masses 1150kg without the added mass of a plug in battery. And that as the powertrain is going to be almost as expensive as on a much larger car, the selling price is going to be horrible on the small car.
The reason that the Leaf, the Ioniq and so on are around the 4.3-4.4 metre mark is because that's what current technology delivers as the minimum practical size based on engineering and economics.