Level 69
Malware Hunter
Avira is lightest based on my tests, using between 50-60MB at idle. Kaspersky is next, using 70-100 at idle. Bitdefender as usual uses almost 200MB at idle (as much as McAfee!). As far as subjective "lightness" tests, I sense the least slow down with Avira, followed by Bitdefender, with Kaspersky feeling the most sluggish (which is not bad compared to some others). Personally I won't use Bitdefender products (or Mcafee) simply because they use so much private memory at idle. I'd go with Avira Pro in your case. :)
In my systen Kaspersky use 55 Mb ram....


Level 3
Avira. Left is Private Bytes and on the right is Working Set in Sysinternals Process Explorer

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Level 40
In terms of lightness Avira i think is lighter followed by Kaspersky and last is BD.
A few months back i tried Avira Pro and it was light on my system.
On the other hand when i tried BD not only it was using about 270 MB of RAM but immediately i noticed that my browsing became slow.
About KIS which i use it right now the 2016 version is very light.No slowdown in browsing and it uses about 40-50 MB of RAM.
But KIS added a few seconds in boot time and shut down time but you hardly notice it.