I've also found RAM usage for Avast is not far off 400 Mb, which is unacceptably high for me, and way higher than the figures in that article. I also found Bitdefender a lot heavier than that article found.
Of the free apps that also have web protection, I found the lightest was Sophos Mobile Security. RAM was about the same as the article found, averaging around 120Mb, but CPU usage was a lot lower than in the article, averaging around 3 to 4% for me.
As for free apps without web protection, I found Panda Mobile Security to be incredibly light. About 6 MB memory usage and pretty much zero CPU usage.