Why I cannot run jobs on the 3Gb Digital ocean servers?

GitLab CI/CD
1

I am running my rails tests on 4Gb Digital ocean servers using gitlab runner. After I changed runner size to 3Gb and restarted coordinator it stopped to run the builds at all

# /etc/gitlab-runner/config.toml

[[runners]]
  ...
  [runners.machine]
    ...
    MachineOptions = [
      "digitalocean-size=3gb" # before there was 4gb
       ...
    ]

No Digital ocean servers are created and jobs show the following information:

Running with gitlab-runner 11.5.1 
  on <Project name> build coordinator <ID>
...

Nothing happens and in the end job fails with timeout

Is it possible to reduce Digital ocean job runner size from 4Gb to 3Gb?