Great question, @shmuga9! There are a few things to consider here.
Currently, Brave (the application) uses more baseline memory than Chrome. What I mean by this is if you are engaging in light usage, Brave is likely to use a bit more. We’re working on identifying areas wherein memory use can be further reduced.
Where Brave begins to shine is when you are engaging in medium-to-heavy usage of the browser, with several tabs and web-apps opened. Brave, by default, blocks third-party ads and trackers, which accounts for a sizeable amount of memory on most popular websites. As a result, tabs tend to be lighter in Brave.
I just took a quick look on my machine at a fresh profile of Brave and Chrome, and Chrome was using ~160MB with a single tab opened (brave.com). Brave was using ~235MB for the same tab. As you suggested, Brave’s built-in security and privacy features do indeed account for some of this different.
I then opened more tabs (brave.com, cnn.com, tmz Google search, twitter.com, and youtube.com), and compared the two again. With just a few tabs opened, Brave was using ~564MB while Chrome used ~708MB. You can imagine the difference between Brave and Chrome for those who have half a dozen to a few dozen tabs opened.
Once you install an extension to handle blocking in Chrome, you have to toss in the memory associated with that extension and its background process (this is accounted for already in Brave’s numbers).
Regarding your question of Why not Chrome with an Ad-blocker extension? It’s important to note that Brave’s content-blocking abilities aren’t derived from the extension APIs offered by Chromium. Extension authors are at Google’s mercy when it comes to what is possible with their extensions; search for the uBlock Origin and Manifest v3 story for an example.
I hope this response helps; if there’s anything further we can cover, please do not hesitate to reach out and ask. We’re always happy to chat!