Hello, Pip. Welcome to TNS.

Surface gravity is dependent on size, but not the overall strength of the gravitational field. For any given mass, the smaller the volume, the greater the surface gravity. But at any given distance from the centre of the planet the gravitational strength will be the same regardless of size.

This is the theory behind black holes. For any given mass, if you compress it to smaller & smaller volume, the surface gravity will increase proportionately (as the inverse square of the distance from the centre); if you halve the radius, the gravitational strength at the surface will quadruple - 1/3 the distance and surface gravity increases 9-fold etc. Continue compressing and eventually there will come a point where the surface gravity is so great that the escape velocity will be equal to the speed of light - c. Photons will no longer be able to escape and the object becomes a black hole.