My guess is that collectgarbage is only telling us about the memory assignment of the lua part of Codea, and that rects are stored elsewhere. I imagine that they’re fairly light thought. Just 4 or 6 points with position and texCoords.
See this link for mesh memory. I’m not sure if this applies or not.
https://codea.io/talk/discussion/2028/adding-texture-to-a-single-rectangle-mesh