Because the strength of a gravitational field depends on how much mass is assembled into a body, and how far away from it you are. For solid bodies, the surface gravity depends on the radius of the body, and its mass ( or density). It is precisely determined by Newton's famous Law of Universal Gravitation:
G M m
Force = ---------
2
r
where G is the constant of gravity, M is the mass of the planet, m is your
mass and r is the radius of the planet, or your distance from the planet's
center.