List<int> to string without enlarging the data file - flutter

Question

I have a compress function like below, which has a return value List<int>

  List<int> compressData() {
    var stringBytes = utf8.encode(plainText);
    return BZip2Encoder().encode(stringBytes);
  }

then I want to encrypt the data with salsa20 from the package encrypt, but must be string data as input data, I've tried with toString() but it makes the data 2X bigger, how to solve the problem?