Here is what I think and keep in mind this is only an interesting theory.

The void hungers and consumes way beyond its ability to sustain itself it is basically life run rampant almost like a cancer. The Shadow Isles is the force of death a force that swells when creature die. The way I see it if the Void didn't essentially overwhelm the Shadow Isles fighting a war would eventually swell the Shadow Isles ranks with the resurrected corpses of the undead and with undead void beast killed in battle and then raised again on the Shadow Isles side. The Shadow Isles would eventually become unstoppable in a long drawn out war of attrition. Also a note while it does say in the lore that the Void has vast armies on other worlds it does not say there are creatures bigger than cities or who eat other planets. It also doesn't say that the Void beasts can eat any magic because if they could how would the magic of Runeterra banish them in the first place? It says there is some ancient and terrible stuff going on but assuming that the Void has whole world eaters is a big leap.

tldr
Void if the rush and overwhelm (short game), Shadow Isles in a war of attrition (long game)

Well" vast armies" implies the void conquered worlds with numbers not through super huge monsters. The lore says they contain his presence to the league of legends but not his size although this is debatable. I mean its not unreasonable to assume he has an upper limit in growth just as EVERYTHING has a limit. Reading the lore it seems less like the Void is sending scouts its more like there are a bunch of monsters constantly scratching and pushing at the barrier to get through and only a few are strong, clever, or lucky enough to find a loophole and get to this side.

Everything has a limit if thats true then why is everyone saying Nasus is limitless.

We don't give the Shadow Isles enough credit. The Shadow Isles has many strengths. First, the world is the Shadow Isles to take. If Karthus ever sings his full Requiem, all of Runeterra's inhabitants could be killed and raised as an army against the void. Also, the undead cannot die. The only reason this "dying" happens in the League is because the summoners force them to create the facade of death and hang back. Not to mention that, but in the long run, by simply killing void creatures the Shadow Isles can create their own void/undead army.

how to compare between the sun and Pluto??

its just like the void and shadow isle..

void is "PLANETS CONSUMED INTO VOID" shadow isles is just.....isles!!