You're comparing lightness with ram usage only which is wrong. There are many other parameters beside ram usage.
AV-Comparative does an excellent job at testing this. Check this: Performance Test April 2019
ESET is the lightest in their test as well so my and many other user's personal experience + this result surely verifies it.
I think I remember other tests that ranked it a bit heavier, but ESET felt very light on my system when I was trying it out. There were no real major system slowdowns or hiccups.