I'm not too familiar with LXQt, I've only used it once or twice briefly, but I know that XFCE has tons of options in regards to customization, including the widgets for the taskbar (called panel in XFCE) and whatnot. Even if it uses more RAM – which I doubt it would be that much higher – as it's not the only metric to consider.
Measuring RAM is also tricky. At idle you can get quite a difference, for example I get ~500Mb on Lite and ~940Mb on Core. However, after some time using it these values tend to even out as the system caches resources to speed things up, which is a good thing.