???
No, that's not why it's arguing that it's the ideal size. Where did you get that idea? (TBH I doubt it's the case - apparently the average PR size on GH is nearly 1000 lines [0])
It's arguing that it's the ideal size because (1) PRs of that size end up getting reviewed and merged the fastest; (2) PRs of that size end up getting reverted the least, implying that they're less likely to cause breakage; (3) PRs of that size end up getting more review comments, implying higher engagement and less rubber-stamping (there's a concern that this could imply unnecessary bikeshedding that slows things down, but see 1); and (4) PRs of that size end up creating the highest total throughput.
I suppose that doesn't "prove" anything, because correlation doesn't imply causation. But it gestures towards it.
[0] https://www.keypup.io/product/average-pull-request-size-metr...
Which IMO is still not worth much or hardly anything at all if we ignore the content of those PRs. I would assume that most are bugfixes or trivial changes (since no tests are needed) so it makes sense that they are causing the least issues.
That’s hardly relevant if you are trying to introduce actual features which require hundreds if not thousands of new lines.
How does that data prove anything? The ideal PR size is 50 lines because that’s the median size on Github. Seem like a pretty worthless claim.