I think there's almost an optical illusion thing with the saucer. It should be somewhere between the two but at that scale it's going to really accentuate whether you are a little over or under. The stairstep nature of the bricks and the relative sizes of things make either stand out. But being too bulbous really stands out.

And it may be better visually to undershoot just a little even if you could mold bricks to the exact right curves, just because our brains fill in the gaps and other models have taught our brains to be pretty generous in smoothing and rounding curves. They often do that where a curve will be flattened a bit because it just reads better.

IMO it's one of the things the other brands tend to get wrong a lot and it gives their models a fat, Macy's Thanksgiving parade balloon look. I agree the Lego one is too flat on close inspection, but I think it reads much better than than bluebrixx which looks over-inflated.