By default, the JVM heap used by Elasticsearch has minimum and maximum size set to 1Gi. As the heap size should not exceed 50% of RAM size, ECK requests by default 2Gi of memory for the Elasticsearch Pod. To change the JVM heap size, use the ES_JAVA_OPTS environment variable. It is also highly recommended to set the resource requests and limits to adjust the Pod memory to not exceed 50% of the RAM size. Both changes are shown below:

spec:
  nodes:
  - podTemplate:
      spec:
        containers:
        - name: elasticsearch
          env:
          - name: ES_JAVA_OPTS
            value: -Xms2g -Xmx2g
          resources:
            requests:
              memory: 4Gi
            limits:
              memory: 4Gi

For more information, see the Elasticsearch documentation on Setting the heap size.