WptAgent docker container is getting bigger

[Daro]

Hi!
I have private instance of WPT in Docker containers. I have done everything exacly as described here.
Running tests is working fine, but container with agent is getting bigger with every test. At the beginning, its size was about 25MB, but after 25 tests it grown to 650MB. I am running tests from command line using node.js wpt wrapper.
Is that a normal situation, and if not, how can I prevent wptagent container from expanding?

EDIT: here are the parameters I call runTest method with:
{ pollResults: 5, timeout: 600, firstViewOnly: true, video: false, runs: 1 }