The force of gravity is proportional to a planet’s mass. So some planets have less gravity than Earth because they are smaller in mass and/or density. Jupiter has the greatest force of gravity (about 2.5 times that of Earth) so you would be extra heavy there!! Many of the planets are composed predominently of gas and so whilst some are bigger than Earth, they are less dense (Saturn and Uranus) and so gravity is less than on Earth.
The gravitational pull on the surface of other planets depends on both the mass and density. The biggest planets in the solar system, Jupiter and Saturn, have gravity much greater than that on Earth. The smaller ones like Mercury, Venus and Mars have gravity less than Earth. But, Mars has a much larger size and mass than Mercury, but their gravity is very similar because Mercury is much more dense than Mars.