I have 8 Gb of ram and I have 7 paritions system. Actually the use of RAM by AVAST has diminished by itself, you are right but after a long time…
But I do not understand the interest of scanning disks (randomly?) after any program installation … Avast does not scan the newly created directory of the new program but scans all over the disk. I can see the input / output paths that avast reads with a program …
And its excessive use of CPU take time, long time …
The Operating System controls the management of the ram but the program (in language C for example) can indicate to destroy objects in memory formerly created and not used.
Optimizing the memory management of a process in memory is already done during development by programmers.
And I’m not saying that Avast’s development team did a bad job, they are very competent !
I explained a clarification on “I was under the belief that your OS controls RAM usage”. 
Thank you for your explication ^^