Mileage is better on the smaller bike
Is it material? I really don't know, just asking.
It sounds like you're not going to be doing a LOT of riding. Even if the mileage is 20% better, if you're only riding 5,000 miles/year (for example), in terms of $, that's not going to be a lot of money.
and I assume some parts are more expensive on the larger bikes. As an example, dual discs on the front of the 400 compared with a single disc on the 200 indicate that front brake maintenance will be twice the price.
I see. Well, I suppose so. But again, if you're not riding a lot to begin with, not sure how material it would be.
But when I look to justify why I'm considering the smaller bike, it's one of several on my list of self-justified reasons.Could be, but the wheelbase is several inches shorter, which should have an impact.
I think your greatest "justification" is probably size and weight. The bottom line is that if a 200 meets your needs, what justification is there to take on more size and weight? Yeah, sure, maybe you would "grow into it" at some point. Absolutely possible. But given your stated use case - doesn't seem real likely.
My car and my son's scoot are already living together in a one-car garage. We can make room for a second scoot, but we expect a tight fit. Even after playing a lot of Tetris with other stored items, too. Yet another justification for a smaller vehicle.
Just ditch the car (: