Why are tablebase files so large? What is the reason that table base files are so large?

I can't think of any other way to phrase that question - but the question alone is rejected by stackoverflow's gateway bot, hence this paragraph added to placate it.
For a simple numerical example, take the endgame of King + Rook + Pawn vs. King + Rook. The number of possible positions is 64 x 63 x 62 x 61 x 60. Of those, 25% is not valid because the pawn is at the first or last row, and because of the symmetry around the d/e file, we only need to store the positions where the pawn is on the a, b, c or d file. This leaves us with 64 x 63 x 62 x 61 x 60 x 0.75 x 0.5 = 343,103,040 positions*.

For every position, you need to know how many moves to mate or conversion, both with White to move and Black to move. You need 2 bytes for each position, so we get a total of 686,206,080 bytes which is about 654 MB. With ZIP-like algorithms, we can compress the size of the files by about 80%, which leaves us with around 150 MB. You can download the files here - note that the tablebases are split between White-to-move and Black-to-move.

For 6- and 7-man endgames, typical sizes are 60 resp. 3600 times as large, so 9 GB resp. 540 GB. Sometimes, symmetry can be exploited further (especially if there are no pawns involved), but this will result in a maximum reduction to 25% of sizes when pawns are involved.

*: actually, some positions are not valid because e.g. both kings attack each other. This number is rather small and if you would filter these, it would take much more time to lookup a certain position in the database.