A hash is a mathematical function that converts an input of arbitrary length into an encrypted output of a fixed length. Thus regardless of the original amount of data or file size involved, its unique hash will always be the same size. Moreover, hashes cannot be used to "reverse-engineer" the input from the hashed output, since hash functions are "one-way" (like a meat grinder; you can't put the ground beef back into a steak). Still, if you use such a function on the same data, its hash will be identical, so you can validate that the data is the same (i.e., unaltered) if you already know its hash.