All AV/AS products will suck up as much RAM is is needed to perform whatever options are selected. Just for basic file scanning none should be all that intrusive, but if you have email protection on, then it's scanning your mail inbox, and that can increase your memory load a lot. Some have many other features, aka cloud scanning, firewalls, that all use up more cycles. I find some products work more efficiently and silently, such as Vipre and ESET, and others appear to steal too many CPU cycles for my liking, aka Norton and Bitdefender. The built in Windows security is actually quite good protection for the price (free), and customizable, though sometimes like any of the other active scanning products I mentioned, can go rogue and take up 100% CPU. Usually this is some conflict with another process that can get resolved with some troubleshooting. Not much comfort I know, but these programs protect us from the evil Internet, and I for one am grateful for what they do.
BTW, Malwarebytes is excellent as the free version and is a must install. It doesn't active scan (unless you subscribe to it) but it as a manual scanner it is nearly full-featured and compliments the built in Windows Security quite well.