Large site optimization is SEO work focused on the systems and templates that generate a site's content, rather than individual pages — since at real scale (typically 5,000+ pages), template-level fixes and automated processes are what actually move the needle, not manual, one-off page edits.
This includes managing crawl budget so search engines spend their limited crawling capacity on genuinely valuable pages, building templated content and metadata systems that produce unique, useful output rather than repetitive boilerplate, and maintaining consistency across a page count too large to review manually.