Long ago, the claim was that the planet Oddworld was 10 times the size of planet Earth, while Mudos was a territory (something between a country and a continent in Earth terms) the size of Nicaragua. (Oddworld History
, both circa 2000)
It's not clear if that's 10 times the size in terms of radius, surface area or volume, but I think the first of those is implied by the talk of speed of technological spread. It's also apparent to me now, unless there's some other source I can't find back right now, that Lorne was talking more about Mudos economically than in terms of size.
Eleven years later, when Nate interviewed Lorne a second time (Nathan Interviews Lorne Lanning Again
), Lorne clarified that Oddworld isn't necessarily ten times the size in radius or
volume, because either way that would cause gravity to be terrible, but in "surface" area: the planet's surface is inhabited, but is actually a shell suspended over a lower layer, itself a shell over an even deeper layer. These layers account for the extra large areas and distances without making the planet implausibly massive.
None of this has been referenced in a game or other in-universe source, so it's liable to change in the future. In fact, even if it is ever confirmed on-screen or in-game, it could be different in another telling of the story.