Logstash take over 1GB memory even though Xms and Xmx are set to 512MB

Question

I have logstash running in a container.

I set env variables (heap size and debug port)

LS_JAVA_OPTS=-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:8083 -Xms512m -Xmx512m

and seems they apply in the JVM settings. Here is the full JVM stats:

"jvm" : {
    "threads" : {
      "count" : 32,
      "peak_count" : 32
    },
    "mem" : {
      "heap_used_percent" : 73,
      "heap_committed_in_bytes" : 536870912,
      "heap_max_in_bytes" : 536870910,
      "heap_used_in_bytes" : 396540928,
      "non_heap_used_in_bytes" : 165070352,
      "non_heap_committed_in_bytes" : 183500800,
      "pools" : {
        "survivor" : {
          "committed_in_bytes" : 27262976,
          "max_in_bytes" : -1,
          "used_in_bytes" : 26738688,
          "peak_max_in_bytes" : -1,
          "peak_used_in_bytes" : 33554432
        },
        "old" : {
          "committed_in_bytes" : 242221056,
          "max_in_bytes" : 536870912,
          "used_in_bytes" : 148552704,
          "peak_max_in_bytes" : 536870912,
          "peak_used_in_bytes" : 148552704
        },
        "young" : {
          "committed_in_bytes" : 267386880,
          "max_in_bytes" : -1,
          "used_in_bytes" : 221249536,
          "peak_max_in_bytes" : -1,
          "peak_used_in_bytes" : 283115520
        }
      }
    },
    "gc" : {
      "collectors" : {
        "old" : {
          "collection_count" : 0,
          "collection_time_in_millis" : 0
        },
        "young" : {
          "collection_count" : 17,
          "collection_time_in_millis" : 1304
        }
      }
    },
    "uptime_in_millis" : 3535183
  }
}

But when I look in top, I see it taking 1.4GB (and keeps growing) in RES, and that's even without doing much work:

If it matters, I have a custom java output plugin which I compiled with logstash according to this tutorial.

What's the reason the heap restriction doesn't work?

UPDATE:

I took a heapdump of logstash and its size is 579861862 bytes i.e. 552 MB.

So if not the heap, what's taking the other 1GB out of the 1.5GB being shown in top?