No hate on the mod or anything, but the planets being the same visual size doesn't contradict anything as we don't know the height of the parking orbit ;)
But it actually makes some sense to park closer to the heavy ones as the cargo rockets would realistically spend much more fuel on the ascent from here.

Ha no worries! This mod came about from this conversation on Visible Planets' mod page:
https://mods.factorio.com/mod/visible-planets/discussion/674c4cc3caedb4f914052b77

I also mentioned there that it doesn't really make any sense and that a ship would probably want to be as close to the planet as possible, taking the atmosphere in to consideration, of course. But in the end I decided I wanted some visual distinction between the planets' scales and when I saw that Nauviax implemented an easy interface to make this, it was only a handful of lines of code and about 2 hours total from starting the project to deploying it on the mod portal. And besides, you can always tweak Visible Planets' "Planet final scale" setting to your liking such that you are really close to even the smaller planets and that the larger ones are even more massive.

If someone was to implement a gas giant, which I've heard some talks about, it would fill your entire screen which seems fitting I suppose.

Yes, like you've said there, realistically you'll want a parking orbit as close to the planet as possible, but since a rocket has the same cost and capacity on every planet one can interprete it as having a higher orbit near the smaller ones to make use of the excess fuel and shorten the trip though the planets gravity well for the platform itself. It's how I like to view it at least. This mod is totally fine of course, if anything, to make things truly "realistic" an appropriate mod should either make rockets cost more on Vulcanus or lower the capacity for such large planets and heighten for the smaller ones.