Several websites claim to be the largest online source for genealogical information but depend on their own definition to establish their claims. Size is always considered a selling point. Think about the giant, economy sizes in your local warehouse store. We tend to be impressed with records of any sort. The Guinness Book of World Records is already out for 2016 and the website has 3804 records for the "largest" things. Some of the largest things recorded include the largest crochet blanket and the largest wearable turban. Size doesn't always equal value; although the largest diamond in the world might be an exception. But I might think someone was grasping at straws to claim the largest collection of stamps featuring Popes.
My rule is that size does not matter if the website does not have the records you are looking for. Notwithstanding my rule, there has to be a certain fascination about size or the websites (or libraries) wouldn't claim to be the largest.
The dividing line between including Google in the claim for being the largest "anything having to do with genealogy no matter how remote the connection" and an obvious genealogy site such as Ancestry.com or FamilySearch.org, is the characterization of the website. In short, does the website purport to be used for genealogy or is genealogical information merely one of the side benefits from having a lot of stuff? Granted, having a lot of records in one place may seem to be an advantage, but if I do a search on a large website for an ancestor and get over a million results, what am I supposed to do with that?
In reality (whatever that is) there is no practical way to compare the size of the larger online, genealogical database programs. So I have observed in the past, there are no clear definitions for the terms records, collections, names, etc. used by the various websites. For example, Ancestry.com lists 32,222 record collections, while FamilySearch.org lists 2036 collections. How do you compare the two lists of "collections" when one collection can contain one record or millions? In addition, Ancestry.com's collections list all of the records they have available. On the other hand, FamilySearch.org has millions of records still in microfilm format that are not yet included in the Historical Record Collections.
Any claims about the total number of records or names in those records must be an estimation at best. I can't imagine anyone sitting down and counting a billion records. Technically, if the individual records were entered into a computer database, the computer database could return a number of total records. That would assume that each individual record was uniquely defined in the database. However, take for example a US Census record, a single record may have 50 names or no names. Here, I use the term "record" to refer to a single U.S. Census sheet or page.
If I have a smaller database, conceivably I could search the entire database manually to determine if there were any records pertaining to my ancestors. With very large databases, we are entirely dependent upon the ability of the search engine to tell us whether or not there are pertinent records available. Although I may search a large database over and over I am never quite certain that I have effectively determined the existence or nonexistence of any specific record. For example, I am not going to search every record in an entire census year simply for the purpose of determining whether an ancestor lived in the United States.
Claims that a database has "billions" of records usually ignore the issue of duplicates. In addition, including user generated family tree entries as "records" obviously obscures the entire claim to a large number of records. My guess is that users will be more impressed with the accuracy of the searches of even a limited number of records than they will be if the searches are too general and thereby less useful.
I recently visited two different stores in one day. One was a large, warehouse store with thousands of products in large quantities. The second store was an extreme contrast. It was a small store with very specialized products. From both stores to be helpful. The interesting thing is that I would not have gone to the large store to purchase the items I ended up buying at the small store. I think genealogy databases work the same way.