If it shows a reasonably accurate preview instead of a flat white, then that image data has to be stored somewhere in memory
I think you're operating under a few incorrect assumptions. 

First of all, an in-memory image always is the same size regardless of what color the pixels are. An RGB image with 10,000 pixels occupies 30,000 bytes. If it has an alpha channel, it occupies 40,000 bytes. I agree that GIF or JPEG FILES on disk can be smaller when all white instead of many different colors, but that has to do with compression, and compression is not involved in BITMAPs, which is how images are represented in memory.

Second, what you see on your screen is stored in the video memory, and particularly in your hardware accelerated preview is calculated and stored in the frame buffer memory of the video card, and regardless of what may have temporarily been consulted to generate the contents of the screen, the frame buffer is always the same size.

Third, under P7 the OpenGL interface can be used (if your card supports it, mine doesn't) to generate per-pixel shading just like or very close to what Firefly renderer does. P7 translates the nodes into shader language for the 3d engine in the video card, and it intereprets them point by point. There is no image in hardware procedural shaders, just like there is no image in software procedural shaders. If your shader uses an image, then that image is downloaded in the video card's texture memory and takes up additional space. Usually the texture is resized to be smaller before loading it onto the card's memory. But if it isn't resized, then it will occupy the same amount of video texture memory that it occupies in system memory. So when dealing with hardware-accelerated image generation (into the frame buffer), the amount of shader data stored in the card is pretty similar to what is needed in system memory for a software-only render.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)