If a planet of given density were made larger, its force of attraction for an object on its surface would increase because of planet’s greater mass but would decrease because of greater separation from the object to the centre of the planet. Which effect predominates?

Increase in mass

F = $\frac{GMm}{r^2}$

If r were doubled Mass becomes 8 times as

$M=\frac{4}{3} \pi r^3 \rho$

Force due to increase in mass increases 8 times and due to increase in radius force reduces 4 times. Therefore, the effect of increase of mass predominates.