I'll take the easier ones out of the way first.
Now, now, Koushiro wasn't stated to be sitting
at the top of the planet. If he was, I wouldn't be complaining about it. There's a slight difference between
on top and
on the top. On top definetely implies he was sitting on the outermost part of the planet. Maybe it has something to do with the translation, I'm not sure.
The basis for all the planets inside being the same size of the one Vademon hurled at MK is this screenshot:
http://prntscr.com/fptfb8 > This is the same planet that was hurled at MK.
http://prntscr.com/fptfrc > The planets are visually identical, the similiarities are way on point for it to be just a coincidence.
http://prntscr.com/fptgxm > Here's how some of the other planets scale to the one hurled at MK.
http://prntscr.com/fpthnu > After being flung a few hundred meters away from Vademon, that's how small it got. Proving that the distance between the planets in my third screenshot cannot be that great after all.
I agree that it might not have been a illusion anymore. However, there are still plenty of evidence that the Mysterious Universe was of reduced size in all aspects:
Koushiro is shown to have no problem breathing in that dimension, even though it's stated to be solar system-sized. If it really was solar system sized, the oxygen would be spread out so that virtually Izzy would not be able to use it as a source of oxygen for his body. Though if the planets aren't real planets and the stars also aren't real stars... I could see why.
Izzy didn't flat out freeze to death, so it's not really a vaccum.
Some of the planets are shown to be relatively close to each other. Usually, they'd smash against the nearest, but these showcase no sign of gravitational pull except for the one Vademon hurls at MK, but since it's a planetoid much smaller than pluto I don't really know what to say except for that it might not be a real planet after all. Just a miniature created by Vademon. Since no density is stated, we cannot make anything out of MK's feat of destroying both the planetoid and Vademon with his Horn Buster.
Weird, I'm sure I can see the Tai kanji there, but it is true that I can't find the Ten. I must've imagined the Ten. Oh well... Sorry about it, though I still have to take a look at the novel that I can't find untranslated for the life of me. It's ironic when you think about, that we can't find the unstranslated version of something when it's usually the other way round.
I also love the fact that Koushiro can dodge the lasers from Vademon's gun. That, or he has the aim of a Stormtrooper, which would make him rather unreliable in battle if he can't shoot down a human and a Rookie.
Ah, yes, yes. There's this too. Wouldn't the shockwave of the explosion of the stars, like, kill the duo? I mean, they can't be far away from him if Vademon was trying to kill them. Even then, the aftermath of the stellar detonations would eventually get to them, right? (the author could be oblivious to that, though)