Re: "So someone decided use different battery size for Model 3 "
If you want distinctly different looking cars, it's very hard to have to base your entire design around a set battery pack. Building different configuration battery packs is not a big deal unless you have designed your production around an automated system that doesn't work. From the tidbits I have seen published and knowing a wee bit about volume production, Tesla was shooting directly for as close to 100% automated assembly of the Model 3 and it's left them with no way to insert human labor in build segments where they haven't been able to work out all of the bugs. I've backed myself into corners before and the first thing to do is admit it and burn some midnight oil to come up with an expedient work-around temporarily. In the case of Model 3 battery packs, it would be creating tools and fixtures that humans can use and setting up some space to do the work. People can be amazingly flexible and easier to implement than robots in the short term.