The attraction force between the Earth and your body is given by
[tex]F=G\dfrac{m_1m_2}{r^2}[/tex]
Where G is a constant, [tex]m_1,m_2[/tex] are the Earth's mass and your mass, and r is the distance between you and the center of the Earth.
In the Earth with the same mass and double radius, the equation would be
[tex]F=G\dfrac{m_1m_2}{(2r)^2}=G\dfrac{m_1m_2}{4r^2}[/tex]
Which is 1/4 of the original weight.