nalimov database for 3, 4 and 5 in FEN format

[syzygy]

Am I overlooking something here?

I'm afraid you're not so much overlooking something as being blissfully ignorant of the basics of tablebase formats.

What all different formats of tablebases have in common is that every possible piece placement has its own, unique index number which represents the position where the information about the position is located in the file.

So chess positions correspond to numbers that index into a file. Given a chess position, you calculate the index and read the value at that position. So the positions themselves are not stored at all. On top of that, most formats use compression. The 5-piece Nalimov tables in uncompressed format are a bit above 30 billion bytes at 1 byte per position. Nalimov only uses 2 bytes per position for certain 6-piece tables. In compressed form the 5-piece Nalimov tables are about 7 GB. As explained, the total number of 5-piece positions is still much bigger than 30 billion, because that number counts mirrored positions only once (and I am guessing you would like to avoid such complications).

Btw, it might come as a surprise that the value stored for a particular position is not the "best move".
https://chessprogramming.wikispaces.com ... Tablebases