Elasticsearch Memory Issue - ES Process Consuming ALL RAM
What you are seeing isn't heap blow out, heap will always be restricted by what you set in the config. free -m and top report on OS related use, so the use there would most likely be the OS caching FS calls.
This will not cause a java OOM.
If you are experiencing java OOM, which is directly related to the java heap running out of space, then there is something else at play. Your logs may provide some info around that.