I really don't know. I would suggest making a blank project with 3840 x 2160 DrawingCanvas, trying to paste some sample sprite on it, save and download. If the result image size is smaller, log it as a bug.
Also, try changing project scaling mode and see if it affects exported image size.
Great Advice dop2000 I just made a clean C3 Test file dedicated to this issue.
I still don't understand why the Exported Resolution is NOT equal to the full Drawing Canvas Object resolution which is exactly the same as the Layout, Both: 3840 x 2160 (4K) as you can see on the file.
Different Tests I tried on this file:
1 - I tried the different project's properties fullscreen mode, Letterbox scale is the best for showing everything while window is changed, also none of the modes FIXED the issue, it was always a WEIRD resolution.
2 - When I have no Background (before I added the Background Layer) the export was always 1920x1080 which is my current display resolution, once I added the Background layer with TiledBackground to cover the full layout 3840x2160 I get a weird resolution on export... strange consider everything is exactly the same resolution? (Drawing Canvas Object, Layout, TileBackground = 3840x2160)
3 - I also tried to change the origin of the drawing canvas to the Center and I re-position it in the middle of the Layout to cover everything perfect, it didn't change anything on export so I undo that to the default origin Top-Left corner position.
I didn't add Camera (zoom in/out) or many extra layers like in my official project to keep it as clean as possible to try tracking down the issue and hopefully understand how to fix it.
THE GOAL:
No matter what the Window Size or if it's Full Screen, ALWAYS: Export PNG to Layout's Full Resolution!
HAVE A LOOK:
File is Attached, if anyone can please explain how to make it work, PLEASE explain or share the new fixed C3 project or a screenshot of the code (easier for noobs like me to follow).
Drawing Canvas - TEST (c3 project file)
Thanks ahead for anyone who can help!