Does the Video Recorder plugin only record based on the actual canvas size, not the fullscreen resolution?
It only records the actual canvas size, because that's the size that is actually rendered. The viewport size is more or less just how much of the layout you get to see, and that is stretched to the window size in the default letterbox scale mode. If the window is smaller than the viewport size, then it will have been resized smaller.
If you want a full resolution recording, make the window fullscreen before starting recording (press F11 or use the Browser 'Request fullscreen' action). If necessary, you can change your display resolution to the intended video resolution.
I'd also note that Construct Animate's 'Export to video' option does record the video at the resolution given by the viewport size - as Construct 3 is a game engine it will just render the canvas fit to the window size.