Digital photography

Old 1Ds, although have a lower megapixel sensor, are full frame 35mm equivalent. And the 7D is a APS-C sensor camera so the sensor is 1.6 times smaller, Although it has more MP, the full frame will have greater detail in its shots.
I have a 6D but I'm not sure how it would do against any of the older 1D range

I understand the differences in the sensor size, but I don't understand how an image with a third the number of pixels can be more detailed? Especially with a large sensor, because that would mean the equivalent area to the smaller APS-C sensor would have an even lower number of pixels (might be mistaken). As I understand it, MPs are a measure of resolution so assuming both cameras have the same lens and are focused perfectly, wouldn't more MPs mean more detail?

The old 1Ds are great - a mkiii should have slightly better AF than a 7D but they're different animals really.
The 6D has only 1 crosstype AF point compared to 19 in the 7D and 7 in your 550D. its more of a portrait/landscape camera.

Its starting to become a bit clearer that despite some cameras being more expensive than others, they might not be as good at a particularly type of photography... The 7d with its fast continuous shooting and more AF points sounds like a better camera for sports.

When you say they're different animals, what's the 1D designed for? I always assumed the top of the range canon body would just be an amazing all rounder?