There may be a low limit on the size of files you can upload. Check your php.ini file. Look for upload_max_filesize
Your php.ini file may be in the root of your site. You can download it and open and edit it with a plain text editor or a code editor.
Some C-panels will allow you to change some parameters of php.ini as well.
Or you can call your host’s technical support and ask them about upload_max_filesize