It is all about the apparent magnification. If for example, you take a 25mm subject, on a 1.6 crop, it will fill the sensor at 1:1 magnification, but it will only fill around 2/3rds of the full frame sensor. Now if you change the subject for a 35mm one, then at 1:1 magnification it will fill the full frame sensor, while on the crop sensor, you can stand further back to fill the sensor, which will of course give you more depth of field. You could crop the image on the 5D MkII, but then you will end up with less pixels, than on the 7D as you know. That said, I also prefer to use the 5D for macros work, partly because it gives a cleaner image (which I feel is more important on macro, than larger wildlife, as fine detail is vital), but also, I find the 5D much easier to focus manually through the viewfinder, giving me a much higher keeper rate when photographing subjects that are likely to move too quickly for a tripod.