I vaugely remember somebody was using MJPEG, on a Mac. Dunno if they used FCP. It can be set to lossless. I don't know the filesize. I don't know if Mac uses codec files that PCs do, like the Morgan Mjpeg Codecs. (Lossy Mjpeg looks horrible, but lossless is, well, lossless).
Googling "MJPEG Mac" turns up a smattering of stuff between the products.