[shkkmo]

It's because volume increases by cubed distance. For a larger planet to have the same surface gravity as ours, it would have to be less dense.

[autokad]

good points. I think the less dense part is pretty 'easy' though, earth is a bit dense for its size. (probably a result of the collision that created the moon).

[shkkmo]

It is completely reasonable to have a planet that is more massive than earth with the same or lower gravity due to differences in density.

However, the density variation of rocky planets is pretty small so as they get larger you will see higher surface gravities.