Hello and welcome.
I’m a little confused about your problem so I’ll need you to help me.
Since you mention LXQt, I’m assuming you’re using Lubuntu 20.04.1 or Lubuntu 20.10. Please let us know if I’m wrong though since some people have come to do some interesting things.
Second, are you saying that your system (as it is now) has seen an increase in memory consumption despite no changes to your system (e.g. updates) or can you recall for us if you performed or encountered an event that caused this recently?
From what I can understand, your currently installed system has seen an increase in memory consumption supposedly and you are comparing to previous memory consumption for the same system with the same Lubuntu version installed.
Incidentally, my own installed Lubuntu (20.04) also happens to run around 600M.