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.
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!