It doesn't refresh automatically, only on demand or (if configured to) when changing folders.
Calculating a folder's size can take significant time and disk access, so we avoid (re-)doing it constantly. There also isn't really a good way to track just the changes to a folder's size in Windows.
There is a tool called Folder Size which adds a column along the lines of what you want which works in both Explorer and Opus. We used to link to that from the Calculate Folder Sizes Automatically FAQ but no longer do, I think because it doesn't work that well with newer versions of Windows and hasn't been updated in some time, but I could be misremembering.