That is expected. You are writing out the re-expanded data as samples. There will be the same number of samples as before, so it is going to take the same output size (probably.)
See also my recent discussion at https://www.mathworks.com/matlabcentral/answers/398289-how-can-i-do-audio-compression-using-huffman-encoding#comment_563731 . For DCT you would not need to write out a dictionary, but you would not write out the coefficients you had zeroed out. You would, however, need to write out the original number of coefficients so when you read the values in, you knew how many zeros to pad with before reconstruction.
