Using a Hough transform to find a circle with an unknown radius is expensive. You have a 3D parameter space, and each point has to "vote" for a 2D subsurface of that space. Discretizing the entire volume at a high resolution is going to require a lot of memory.
I agree it is going to expensive but it's not because of memory. You could probably just try every (x,y) center and have all points vote for the radius, and just keep max before moving on to next point.
