But the width of those mountains is already comparable to the width of Zou (10km).
Nah bro, they're just half the width of Zou at best, look carefully again:
And Zou makes up Zunisha's entire back, so the width of the mountains are not exactly comparable to Zou.
Again, I fail to realise why those mountains make Zou's size look inaccurate; those are also quite large in comparison to Flower Capital which is a city.
That means that the diameter of the flower capital should be ~50km
Nope.
Flower Capital might be ~20km or even ~25km. Zunisha standing straight on front of it might give an answer.
It looks like a village at most.
???
I don't see how it looks like a village, though. Especially if we see these huge mountains on the main-land.
The real Mount Fuji for example has a height of 3,8km. Going by the calcs however, the Mt. Fuji in One piece would be like 200-300km in height.
Well, only because the real Mt Fuji is 3.8km doesn't mean One Piece's Mt Fuji has to be either. Even before the Zunisha + mountains comparison, Mt Fuji was estimated to be ~100km in height easily since it is several times larger than Onigashima. But now with Zunisha's height being more comparable to the mountains whilst the rock structure is half of Zou in width, that's actually something new which can be used.
Also, One Piece's world is so much larger than ours. Countries like Alabasta have the size of Australia, it's mind blowing. You'll see when we finally see Elbaf, the country of the giants.
The same with OP's outer space; we don't know where even the stratosphere starts. Onigashima is very high up the sky and there's still no clear sight of the outer space. Thus, it could be far larger than ours as well.