I am reading a file by using:
int len = (int)(new File(args[0]).length());
FileInputStream fis =
new FileInputStream(args[0]);
byte buf[] = n
define "significantly". in java, an int is 4 bytes, so by definition the array would be 4x the space. See: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
And during the conversion, you have to have both, so during the copy portion, you'd be using even more, if you were copying the whole array at once.
as for the conversion, there are many related questions:
Java - converting byte array of audio into integer array