The statement "Oddworld is ten times the size of Earth" has been open to debate since Lorne first stated it. Is this volume? Diameter? Surface area? Content? Metaphorical? I believe it was said that any new technology or cultural movement would take ten times as long to spread around the world as it would on Earth. Whether this is due to the distance, the conservative nature of the inhabitants or something else is not clear. The Glukkons have used fat as a highly inefficient fuel source for hundreds of years, they are resistant to change.
If this were a physical distinction, there would have to be a number of reality suspensions or elaborate physical anomalies to ensure the environments we have observed on the planet. In terms of gravity, one unusual theory I heard was that there was an exotic ore that exerts the antigravity force, used in security orbs, that counteracts the expected gravity. Another is that the planet is less dense than Earth and therefore has less mass to exert gravity, but given the planet's size this would not happen. Plate tectonics is also an issue, and it is required for there to be life.
The size of a planet does not affect its year length- that is related to the distance from which it orbits its star. We don't know what kind of star it is, so we don't know how far out the habitable zone is. Size does affect rotational speed though- the day length. Larger planets tend to spin faster, so an Oddworld day would therefore be much shorter than an Earth day. However, the tidal effects of the moon slows the Earth's rotation to its present speed, so all of Oddworld's moons and the elaborate tidal system they create could reset the day length to a more familiar one.
I think the ages and life-expectancies on
Oddworld.com are probably in Earth years for convenience.