@Fla no the Android layer is not the reason, this is very little RAM usage. The problem is that unity8 was never optimized for low-memory devices without swap: It comes from a desktop world, and with it a lot of services that just like to spend RAM.
Unfortunately the core team has no resources to tackle those issues with unity8 right now, and the Yunit project, which wanted to fork and maintain unity8, has silently died away. So currently we can only be patient with that situation.