I am running the Muon Package Manager (version 5.7.0) on KDE Neon User Edition 5.10. After right clicking on any of the column headers there is a menu where one can select additional columns to be viewed ("Installed Size", "Installed Version" and "Available Version"). After selecting some of these options the extra columns are shown as expected. If I quit and restart then the additional columns are not remembered and I have to re-enable them again every time I start Muon.
Muon is no longer maintained, please switch no a supported alternative like https://apps.kde.org/discover/ or the package manager native no your system. Sorry for the inconvenience.