Screenshot size Base64

I take a screenshot of a part of a web site and transform it into a Base64 string.
I’m including the string in a json body in a HTTP POST (api call).
The problem is that the string is too large to be sent as POST.
Error message from HTTP Request activity:

the Base64 string is length: 1045552 which I think converts to almost 1MB.
The API I’m calling can take 10MB.
This works on smaller screenshots. Any suggestions appreciated!
I don’t know why the screenshot image is too large for a POST.