Memory pressure during recursive directory size calculation

I'm only running the following command and here are my resources available:

```
s4cmd du -r s3://mybucket
```
A couple hours in.. this is a big bucket but why does this tool need to allocate so much resources to do its job?

Start by reproducing `s4cmd du -r s3://mybucket` with a large bucket and trace the recursive directory-size calculation used by the `du` command. Done means the command no longer requires excessive memory while calculating a large bucket's size; the issue names no files or tests to guide further investigation.

Written by the indexing model from the issue text.