This is how the _changes
api is designed. If you do not specify the type of feed i.e long-poll, continuous etc the default is to return a list of all the changes in a single results array.
If you want a row by row result of the changes in the database specify the type of feed in the url like so
curl -X GET "http://localhost:5984/mydb/_changes?feed=continuous"
Another point to note that in the _changes api using 0 has the same effect as using 1 in limit parameter.