Problem with VPS memory usage while still no subsites

Hello,
I have a multisite install, so far with no subsites, installed on a VPS with 2 Gb guaranteed and up to 4 Gb memory. But the weekly and daily reports both indicates a use of 3,93 GB...

I don't want to start adding subsites if my config is not right. What should cause what seems to me a problem?

Let me know,
Julien