[Backport 2.x] Fix graph merge stats size calculation

[ryanbogan]

Description

Backport of https://github.com/opensearch-project/k-NN/pull/1844

Check List

• [x] New functionality includes testing.
• [x] New functionality has been documented.
• [x] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [x] Public documentation issue/PR created.

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