Going back to this, I think it's worth pointing out that comments on the size of the planet might not be entirely valid. Whilst the planet may appear around Wario's size at first
due to the visuals, to say this is actually the case seems to be forgetting the context of the feat. This planet is seen as a better reward than two
large castles that visibly have multiple floors and rooms, and so are clearly much larger than Wario. For this planet to actually be tiny in-universe would make it more disappointing than these grand living spaces, so to actually believe it's a planet Wario couldn't even use as a place to live would defeat the point of it being the "best ending" to the game.
And yes, this doesn't necessarily prove the planet is an entire planet from there (the sheer difference in size between the castle and a planet is indeed huge), the fact we see cloud cover on the planet does imply a large enough size to sustain ecosystems. To add to this, you can simply see it as "a planet is a planet, you can't say it's only half", where the simplest conclusion is that when we're shown an Earth-like planet, the narrative intent was that it was indeed a planet, the visuals simply toning it down so it can show what it wants out of the cutscene on limited hardware. It's not uncommon in fiction for someone to be seen as much larger than a planet they threaten dominion over for a cool visual, Perfect Kars from JoJo being an example that comes to mind. (Whilst, yes,
Galaxy does show small planetoids aren't unheard of in this franchise, that game established this many years after Wario Land came out, so there wasn't the tone set for such a concept just yet. If we were scaling this game when it came out, we wouldn't have that argument to fall back upon yet, so why assume the scene itself would consider that?)
I suppose just something I wanted to point out since it seems to be a recurring counter to the feat without actually looking deeper into the details surrounding it.