Odd Memory Usage

Hi!

I've been using this library for a while and noticed an unusual behavior regarding memory usage. When my consumer receives a lot of messages, the Node.js application's memory usage naturally increases. However, I've observed that while heap usage decreases after processing the messages, the .rss (resident set size) memory remains significantly higher than expected.

Here is an example of what I'm experiencing:

- **Initial Memory Usage:**
RSS: 200 MB
Heap: 50 MB
- **During Message Processing:**
RSS: 1.5 GB
Heap: 500 MB
- **After Processing:**
RSS: 1 GB
Heap: 60 MB

Is this the expected behaviour?

Thanks!

No files, tests, or entry points are identified in the issue. Start by reproducing the reported memory pattern in the Node.js Kafka consumer and inspect the relevant node-rdkafka and librdkafka behavior; done means establishing whether the RSS retention is expected and documenting or isolating a reproducible defect.

Written by the indexing model from the issue text.