The current implementation of RestVerticle inside of RMB contains the method
private void handleInputStreamUpload(...)

This part of the code is Buffering all body content that was sent chunked into the UnpoolHeapBuffer. So if we try to upload 1Gb file it needs 1Gb of the java heap memory.

There are two possible solutions:
Store uploaded file into a temp file and create a FileOutputStream and push it inside of the API method or just push HttpServerRequest inside API method and read file chunks manually.