Forget about the compression, my initial question is not intended to put it as a consideration.
I apologize to everyone who is confused on the topic title.
I try to rephrase it.
Let say I already have an API
https://mydomain/update_order_status/v1
which accept JSON as body. The body may be very long string.
Now, I want to create a version v2 which accept the body in bytes instead. If I understand correctly, the data would be the same as what I get when using v1 except for I need to transform back the bytes into JSON. In this case, there is no difference at all or any point creating a v2.
The question is not really about PostString vs PostBytes. Actually I want to know the best practice to create an efficient or a better way to build the web API.