For instance: 'diagnosis' gives back > 7 million results and that is roughly 1/3 of all the citations. I know that I am probably asking an 'irrelevant' search question as I am after the biggest scope, the inverse of relevant search.
If you query for 'publication formats' you get essentially all of PubMed back. The reason is that their automatic query turns extends this query to "publication formats"[Publication Type] OR "publication formats"[All Fields]. Almost entry in PubMed has a publication type, which is a child term of the MeSH term 'Publication Formats'.
The reason why 'diagnosis' gives as many results as it does is much the same. The query expansion turns it into "diagnosis"[Subheading] OR "diagnosis"[All Fields] OR "diagnosis"[MeSH Terms]. The latter MeSH term is one of the top-level terms.