Justin Tobler [off-list ref] writes:

Complex merge events may produce an octopus merge where the resulting
merge commit has more than two parents. While iterating through objects
in the repository for git-repo-structure, identify the commit with the
most parents and display it in the output.

Does the size of octopus have anything more than a curiosity value?

The opposite, the commit with most direct children, might be even
more interesting, but that may be just me.