Neptune OpenCypher insert relations throw OOM frequently

The instance is with 64GB memory, I'm calling write_rels as shown below, which is basically inserting about 500~1000 relations into the Neptune concurently, but i keep receiving this error

{code: "BoltProtocol.unexpectedException"} {message: "Unexpected server exception 'Operation terminated (out of memory)'"} 

I would like to know why this happens and how to avoid this and insert relationships more effectively.

def write_rels(rels: list[dict], group_size = 200, concurrency= 16)->float:
    print("write_rels, rels size:", len(rels), "group_size:", group_size, "concurrency:", concurrency)
    d = Driver()
    groups = []
    for i in range(0, len(rels), group_size):
        groups.append(rels[i:i+group_size])

    start = time.time()
    with concurrent.futures.ThreadPoolExecutor(max_workers=concurrency) as executor:
        futures = []
        for data in groups:
            futures.append(executor.submit(d.batch_create_rels, data))
        for future in concurrent.futures.as_completed(futures):
            pass
    end = time.time()
    return end - start
    

class Driver():
    def __init__(self) -> None:
        uri = ""
        self.driver = GraphDatabase.driver(uri, auth=("username", "password"))
        self.check_connectivity() 

    def batch_create_rels(self,data):
        def merge_nodes(tx):
            query = f"""
            UNWIND $data AS param
            MERGE (n:{label} {{id: param.id}})
            ON CREATE SET n = param
            ON MATCH SET n += param
            """
            tx.run(query, data=data)
        with self.driver.session() as session:
            return session.write_transaction(merge_nodes)