RDP / Share screen bigger than actual size?
In short, you can't.  

There is a fundamental difference between VNC and RDP.  VNC is a remote frame buffer whereas RDP is more semantic in nature and fully aware of the underlying desktop.1

RDP actaully creates a desktop on the client machine and interprets handles fonts, pixels and resolution.  VNC just gets the pixel data.  This is also why VNC is horrible over anything but LAN connections whereas RDP is very efficient over even 56K dial up.  Repainting a bunch of pixels to move a window is vastly more inefficient that sending a command that says "draw window of size (HxW) at coordinates (X,Y)"


  The VNC protocol is pixel based. Although this leads to great
  flexibility (i.e. any type of desktop can be displayed), it is often
  less efficient than solutions that have a better understanding of the
  underlying graphic layout like X11 or desktop such as RDP. Those
  protocols send graphic primitives or high level commands in a simpler
  form (e.g. open window), whereas RFB just sends the raw pixel data,
  albeit compressed.


So, while your iMac is capable of much higher resolutions, the RFB protocol will only send pixels that are being displayed and your MBP or VNC can't create pixels where none exist.



1 RFB protocol. - RFB (“remote framebuffer”) is an open simple protocol for remote access to graphical user interfaces. Because it works at the framebuffer level it is applicable to all windowing systems and applications, including Microsoft Windows, macOS and the X Window System. RFB is the protocol used in Virtual Network Computing (VNC) and its derivatives.