Interesting question. As usual I can't give a simple answer. "Meat Bird" doesn't always mean the same thing to everyone on here. To me there are three different types of meat birds.
1. They might be called Cornish X, Cornish Cross, or Cornish Rock. It is not really a breed but more of a type but that is just being precise, doesn't really matter. This is the chicken you probably buy at the store. As mentioned above, excellent meat production, often ready for butcher at 6 to 8 weeks depending how you feed and manage them. You do not raise these for egg production, they are the meat specialists. They grow so fast it can be hard to keep then alive past 8 to 10 weeks of age.
2. Often called some kind of Ranger but can have other marketing names. Also not a breed but a type. They develop slower than the Cornish Cross and were developed to take better advantage of pasturing. They still are a very good meat bird. You also do not raise these for egg production. They are meat specialists but not as much and are more durable than Cornish X.
3. There are so many different breeds of Dual Purpose I'm not gong to even start mentioning them. These are the more typical "homestead" or "farm" chickens you often see on TV or in movies that farmers raise. They provide a fair amount of eggs and a fair amount of meat, but are not anywhere close to the efficiency of the Cornish X or Rangers for meat or the egg laying chickens for eggs. Just a good all-purpose chicken. I raise these as my neat birds
Nor all dual purpose are breeds either. Many of us raise mixed breed chickens as dual purpose. Some might be crosses of specific breeds, some may be what we often call barnyard crosses or mutts. Dual Purpose covers a lot of territory. To complicate it even more, with many dual purpose breeds you may have full sized chickens or bantams. Bantams are miniature versions of full-sized fowl of that breed. If your Cochin and Orpington are full-sized they would be considered dual purpose. If they are bantam I would consider them decorative. They still lay eggs and you can eat them, but that's not what people would typically raise if meat or eggs were the only considerations.
Then there ae those not considered meat birds. The Commercial Layers are also not a breed but a type. They have been developed purely for egg laying. They are excellent at converting what they eat into eggs. Their small bodies do not produce much meat. Some breeds are better for laying than meat, like the Leghorn.
Another broad category are the decorative breeds. They were developed more for beauty or uniqueness than egg or meat production. Many are bantam but certainly not all. Some might have strange feathering like Silkies or the long tail like the Phoenix. Some may have strange features like the Polish crest around the head.
You can eat any of them. They all can lay eggs or there would be no eggs to hatch but some of these don't produce much meat and do not lay many eggs and some lay really small eggs. These are very broad categories. There is a lot of crossover between them. There are specialists, like the roosters raised for their feathers to make fishing lures. You just can't cover them all.
When someone on here talks about "meat birds" I don't always know what breed or type they are talking about. You might notice some of us ask for clarification because you do raise and manage them differently. At the end of the day all these are chickens but, man, what a variety!