So, I visit this question after 1 year anniversay of my failure of upgrading Big Sur in my Macbook Pro 2014 128GB SSD version. I successfully download the Big Sur but failed once again to install it because of extra 12GB reguired. Quick and easy solution. Go to Application Folder, move the "Install Big Sur" app (which is 12.45GB) to an external harddisk. Start the installation from the external harddisk Problem Solved.