The equation to graph a nice set of breasts in 3D space.

z=x4 + y4 - 4xy + 1

If you're using this equation for the Tomb Raider Model, the third term must be increased with the release. Thus the breast of Tomb Raider 1 will be smaller than Tomb Radar 2 and so on. It's amazing what 5 college calculus class, some free time, and a dirty mind and come up with.
This doesn't do a very good job of simulating gravity. You might want to use this as a starting mesh to put into an inverse kinematics engine. Also, you don't specify the constraints on x or y.

What might have better results would be z=x4+(y-gz)4-4x(y-gz)+1, where g is the amount of gravitational skew. Yeah, I know, that's not a nice heightfield equation. Deal with it. Of course, it's simple to view it as a 5-dimensional isosurface instead:

x4+(y-gz)4-cx(y-gz)-(z-1) >= 0

where g is the gravitational skew, and c is the cup size constant (4 to start with). This should maintain the property that the surface only exists for z >= 0, though I don't have an isosurface renderer handy.

A better breast equation, however, could be this: whistles + catcalls + dehumanization = K * breast size - the bigger the breasts, the more a woman is likely to be treated as an object.