Your computer probably shouldn't be using that much RAM. It might be a memory leak, in which case adding more memory might not even help much. A quick search turned up this thread, where some others were having similar problems in Overwatch...
https://eu.battle.net/forums/en/overwatch/topic/17611941925
One of those affected found that at least in their case, it was due to a memory leak in some software for their network adapter, and linked to this thread here, where a number of people posted about having similar issues with the "Killer Network Suite" filling all their RAM...
https://www.reddit.com/r/buildapc/comments/3yzjyz/killer_network_suite_gave_me_a_major_memory_leak/
If your motherboard has a Killer network adapter, that could potentially be the problem, otherwise it might be some other piece of software causing a similar issue.