Hi Burr, well the base and target surfaces are what controls it.

It's kind of like texture mapping - objects are mapped from the UV space of the base surface into the UV space of the target surface.

If the target surface has areas where the UV space is compressed or expanded then the shapes will compress or expand in those areas as well.

If you need to have a whole lot of control over it, it may be that you would want to more directly model the output shape rather than using the Flow tool to morph it. But if your target surface is pretty well behaved (like if it doesn't pinch or expand irregularly) then the Flow command should be pretty convenient to use.

- Michael