Delphi encrypted file is much smaller than the original?

Delphi encrypted file is much smaller than the original?

I am loading a binary file into a memorystream, then encrypting the data,
and returning the result as a string, then writing the result into another
memorystream, and saving it to a file, but when it saved the file is much
smaller than the original 25kb from 400kb...lol, im pretty sure it's
because I've hit the limit of what a string is capable of handling.
it's definately encrypting what data it does save in the new file
correctly, I decrypted it and compared it to the begining of the original
file.
I know this is a very long winded method and probibly has some unnecesary
steps, so loading it into bStream would be a very effective resolution. My
question is how could I have the data returned to bStream rather than
having it returned to a string then writing the string to bStream at that
point as I do believe, that it would solve my problem, any other
suggestions would also be appreciated. Im using Delphi 6.
Heres My Code:
function B64Encode(pInput: pointer; pOutput: pointer; Size: longint):
longint;
var
i, iptr, optr: integer;
Input, Output: PByteArray;
begin
Input:= PByteArray(pInput); Output:= PByteArray(pOutput);
iptr:= 0; optr:= 0;
for i:= 1 to (Size div 3) do
begin
Output^[optr+0]:= B64[Input^[iptr] shr 2];
Output^[optr+1]:= B64[((Input^[iptr] and 3) shl 4) + (Input^[iptr+1] shr 4)];
Output^[optr+2]:= B64[((Input^[iptr+1] and 15) shl 2) + (Input^[iptr+2]
shr 6)];
Output^[optr+3]:= B64[Input^[iptr+2] and 63];
Inc(optr,4); Inc(iptr,3);
end;
case (Size mod 3) of
1: begin
Output^[optr+0]:= B64[Input^[iptr] shr 2];
Output^[optr+1]:= B64[(Input^[iptr] and 3) shl 4];
Output^[optr+2]:= byte('=');
Output^[optr+3]:= byte('=');
end;
2: begin
Output^[optr+0]:= B64[Input^[iptr] shr 2];
Output^[optr+1]:= B64[((Input^[iptr] and 3) shl 4) + (Input^[iptr+1] shr 4)];
Output^[optr+2]:= B64[(Input^[iptr+1] and 15) shl 2];
Output^[optr+3]:= byte('=');
end;
end;
Result:= ((Size+2) div 3) * 4;
end;
procedure TForm1.Button2Click(Sender: TObject);
var
aStream, bStream: TMemoryStream;
strastream: string;
szaStream: integer;
begin
bStream := Tmemorystream.create;
aStream := TMemoryStream.Create;
aStream.LoadFromFile('C:\file1.exe');
szaStream := (astream.size + 2) div (3 * 4);
setlength(strastream, szaStream);
B64Encode(astream.Memory,@strastream[1],Length(strastream));
bstream.WriteBuffer(strastream[1], szaStream);
AttachToFile('C:\file2.exe', bStream);
bstream.Free;
aStream.Free;
end;
Thanks.