Check your Folder Formats, on their Options tabs.
Each Folder Format can set get folder sizes to default (does whatever is set in Preferences) or automatic (always calculates, regardless of what Preferences says) or off (never calculates, regardless of Preferences).
(I'm not really sure why the option is in both Preferences and Folder Formats. It seems redundant to me, but I guess when all of the Folder Formats are set to Default it does provide a quick and easy way to turn the feature on, off, or on for local drives only.)