Second...is the 600 truly a 600 or is it a 570mm? ....
You do know all lenses are rated in MM @ Infinity focus right? It's the magnification that changes the "appeared" MM when you are at MFD.
My Tamron 70-200 is actually slightly longer than the Canon 70-200 2.8 MKII when both are focused at Infinity. But when both are focused at MFD the Canon is much larger due to it's .21x magnification compared to the Tamron's .13x
Just wanted to let you know because it seems you put that into a factor. You need to look at the magnifications of the lenses too.