I've never heard Ubuntu claiming to be lightweight.

Ubuntu's initial claim to fame was a normie (non-techie) could install it and the stuff they wanted to work would Just Work- there'd be a little option during install 'do you want to install closed source or proprietary or patent encumbered shit?' and you'd click yes and then your graphics card would work and your MP3s would play and your sound card worked without having to fuck around with kernel modules and command lines.

If you want lightweight install Xubuntu (xfce environment).