I haven't tried sending data that large before. First, are you configuring the REST proxy with the necessary client configs (e.g. max.request.size) to allow large messages? The default max message size is much smaller than 20MB. You'll also need to make sure you're topics are configured to accept messages that large.
If that is all correctly configure, my next guess is that you're running into Jetty's default limit on request size. I only did a quick search so I'm not certain this is the correct config to be looking at, but
https://www.eclipse.org/jetty/documentation/9.2.7.v20150116/configuring-form-size.html suggests that the limit is 200K (I'm only unsure because this actually isn't a form, it's just the request entity size).
Unfortunately, if that's the issue (which you can probably find if you turn up the log level or maybe turn on debug=true to get the error message returned in the response), you'd need to adjust the Jetty config and that will require recompiling the REST proxy at the moment.