Systems and methods for reducing file sizes for files delivered over a network are disclosed. A method comprises receiving a first file comprising sequences of data; creating a hash table having entries corresponding to overlapping sequences of data; receiving a second file comprising sequences of data; comparing each of the sequences of data in the second file to the sequences of data in the hash table to determine sequences of data present in both the first and second files; and creating a third file comprising sequences of data from the second file and representations of locations and lengths of said sequences of data present in both the first and second files.