Dorking are not large birds. They are said to be a good meat bird due to quality/proportions of fleshing and fine texture of meat. That has nothing to do with size but Dorking are considered fast maturing so have a lot of size at young age. As Dave C said you select for fast maturing birds not adult size. Broiler age of birds is 12 to 14 weeks. It's said you can broil/grill up to 16 weeks but will be tougher and cull my cockerels week 14 or earlier to prevent that.
Cornish will be larger and also have excellent proportions. Their meat is coarse compared to Dorking but certainly not as coarse as Brahma or Jersey Giants.
It would have more to do with the line of bird than the breed. Fat Daddy has an excellent line of White Cornish. Has a thread going on them and performance. Can't remember the people that keep Dorking. You'd have to do a search and find any weights given. Each line is going to vary on rate of maturing.
I mean if you get hatchery Cornish they won't perform as well as a good breeder line of Dorking. It has a lot to do with where the birds came from.