is there a way to reduce the file size without changing the canvas size?

Video Compression requires an even number of horizontal and vertical rows of pixels. You can't compress color if you have an odd number of lines because color compression averages the colors in blocks of 4 pixels. There are also specific frame sizes allowed with certain formats. Change your comp to an even number of horizontal and vertical pixels, and the warning will be removed.

You should also be aware that streaming platforms have specific frame sizes. Ensure you are following the platform's guidelines (YouTube, Instagram, Vimeo are just a few examples). Most media players also have limitations on the frame sizes they support. You may end up with a video that has black borders when played back if you don't follow the standards.