That’s correct. Clamd uses abot 1/2 a GB. I think the 1.5 GB ram is bit low. A real system would have more ram nowadays, wouldn’t it? 
That may be true for Centos. Either Centos has a pretty big foot print or it has to do with the concept of being an all-in-one server. For a pure gateway 1 GB is enough, look at this screenshot.
comparing linux (centos) and, more precisely, NS, with pfsense (BSD) makes no sense at all
That’s true even for CentOS/NethServer.
But some services need a lot of ram, regardless where they run on Linux or *BSD?
Take clamd: if you load all sigs, you need around 0.5 G of ram.