My best guess is you have a lot of apps that open on startup that might be the issue but when you have more ram, you pc naturally allocates more ram to the OS to ensure smooth functionality, for example when I upgrade from 8 - 16 GB macOS went from using 4 GBto 6 GB maybe thats why? Also it could be some app using high memory like video/ photo editor. These are guesses at best tho
My best guess is you have a lot of apps that open on startup that might be the issue but when you have more ram, you pc naturally allocates more ram to the OS to ensure smooth functionality, for example when I upgrade from 8 - 16 GB macOS went from using 4 GBto 6 GB maybe thats why? Also it could be some app using high memory like video/ photo editor. These are guesses at best tho