Xperia M has 1GB of RAM available for user and system use.
I have approx 300MB of RAM free(700MB used).
If you only have 100MB of free RAM or you're constantly using 900MB of RAM, then there is probably issues that need correcting.
When you're only seeing approx 600MB/700MB of RAM used, you don't have a problem.
Android will pre-load applications into memory or leave them cached in memory to speed-up application startup.
Android manages memory (RAM) for you.
So no matter how much you "end tasks", they'll just keep reloading back into memory if there is free memory (RAM) avaiable.
That is why many people frown on Google Play Store "task killer" applications.
Applications like Greenify(probably one of the better ones) attempt to manage application usage patterns and maybe even more beneficial, attempts to reduce battery use.
I have root access on my phone and have deleted many system files like facebook, twitter and tens of others.
This can help by eliminating candidates which could/would be cached/active in memory.
When you don't have root access you can disable applications like facebook within settings->apps.
My pet hate when it comes to RAM is applications you downloaded that only get used once or twice a week and yet they hold down a 20/30MB service in RAM... I hate that. Most good applications don't do this, but some poorly written applications still do. (Mail, calendar, Home, keyboard and etc would be expected to have a constant RAM service, but not a TV guide for example)
Google Play Store app - SystemPanel, is a good app for viewing what the system/RAM is doing. .. and who's doing it to you.
