One throwaway simple answer that would work in a book:
The other day I was thinking, imagine if our Earth was just a bit bigger - say, twice the size (twice the radius).
It occurred to me:
the surface area would be massively, spectacularly, stunningly bigger - imagine an Earth with four times the surface area.
our present Earth has the three or four land regions - North America, Sth America, Africa, Eurasia.
Bearing in mind too that the poles are a bit wasted, an Earth merely twice the radius would have perhaps 20 or even more such "massive socio-political-historic regions".
About 10x the landmass and 10x the population would make it absurdly difficult to conquer. Your characters would talk about how it would be like conquering a whole system of 10 ordinary planets...
Consider all the amazing modern history, say last 400 yrs, of Africa.
All the ins and out, different "eras", colonialism, etc etc...
On this planet we really only have three or four "major stories," like that.
Consider, if Earth was a "large surface area" planet, with merely twice the radius, the overall human story would be just really astoundingly large and complex.
There would be perhaps 15, 20, massive "major areas" each with substantially massively different cultures and stories.
Let me explain how I came to this thought:
On this Earth as we know it, I've been everywhere! :) There are maybe 10 good bars and I've gotten smashed at all of them. I was thinking ... imagine if we had half a dozen Earth-like planets all close in our system and travel was possible. That would be fantastic! Instead of just "Asian culture", "African culture", "The Americas", and "European culture" - with these half-dozen planets, there would be 10, 20, 30 such massive major regions. Instead of the 5 or so major world megacities we know, there would be 30 or 40!!
All of that is great, but then I realized, if you simply had a planet that was merely twice the radius, that is such an incredibly large surface area that indeed it would be just as good as having a half dozen Earths!
As a typical evil villain megalomaniac, given some advanced alien technology i can sort of imagine "conquering all of Earth" as we know it....
But if Earth had 10, 15 massive major regions - rather than just the 3 or 4 we have currently - it would be a whole other thing.
And many, many more good bars.
(Of course, watch out for gravity - but you can handwave that, particularly as we already have terraforming, psychic power, and FTL. It's precisely the sort of thing that would make the planet special.)