You may be confused by the visual representation of a rubber sheet. The rubber sheet analogy is only a visual representation of gravitation outside a massive body. As gravity is proportional to mass, not size, the analogy becomes awkward if applied to a large hollow object. A hollow object affects objects outside it, as though all its mass were concentrated at a point in its center. Objects inside it can not so easily be accommodated by the rubber sheet analogy.
It may help your understanding of why the rubber sheet analogy is inappropriate in some cases if you consider what happens gravitationally inside a hollow massive object. Take a look at this website:
http://www.grc.nasa.gov/WWW/k-12/Numbers/Math/Mathematical_Thinking/grvtysp.htm
Also, see the wikipedia article on the Shell Theorem:
http://en.wikipedia.org/wiki/Shell_theorem
Isaac Newton proved the Shell Theorem, which says that (1) if a massive object is a hollow sphere, it's gravitational effect on external objects is as though all its mass were concentrated in the center of the sphere, and (2) no net gravitational force is exerted by the sphere on any object INSIDE, regardless of the object's location within the shell.
An interesting corollary to this is that if there were a gravitating body which could somehow move unobstructed in a hole through the center of the solid Earth, it would spend its time oscillating from outer surface to outer surface through the center. At least, this is what I got from a Leonard Susskind lecture on gravity. See this website for a "hole through the earth" example:
http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/earthole.html
Don't forget that where gravity is concerned, size makes no difference!