program Project1;

uses

 SysUtils, Classes, ZStream;

function ZCompressStr(const AString : UnicodeString; ALevel : TCompressionlevel = clDefault) : TBytes;
var
 Stream     : TBytesStream;
 Compressor : TCompressionstream;
begin
  Stream := TBytesStream.Create;
  try
   Compressor := TCompressionStream.Create(ALevel, Stream);
   try
    Compressor.WriteBuffer(Pointer(AString)^, Length(AString) * SizeOf(WideChar));
   finally
     Compressor.Free;
   end;
   Result := Copy(Stream.Bytes, 0, Stream.Size);
  finally
    Stream.Free;
  end;
end;

function ZDecompressStr(const ABytes: TBytes) : UnicodeString;
const
  Growth = 256;
var
 Stream       : TBytesStream;
 Decompressor : TDecompressionstream;
 Count        : LongInt;
begin
  Result := '';
  Stream := TBytesStream.Create(ABytes);
  try
   Decompressor := TDecompressionStream.Create(Stream);
   try
    repeat
      Setlength(Result, Length(Result) + Growth);
      Count := Decompressor.Read(PByte(Result)[(Length(Result) - Growth) * SizeOf(WideChar)], Growth * SizeOf(WideChar));
    until Count <> Growth * SizeOf(WideChar);
    Assert(Not Odd(Count));
    SetLength(Result, Length(Result) - (Growth * SizeOf(WideChar) - Count) Div 2);
   finally
     Decompressor.Free;
   end;
  finally
    Stream.Free;
  end;
end;

begin
 WriteLn('"', ZDecompressStr(ZCompressStr('Hello, ZLib!')), '"');
end. 

var s : UTF8String;
ZBytes : TBytes;
begin
 ZBytes := ZCompressStr('some text');
 SetLength(s, Length(ZBytes));
 Move(Pointer(ZBytes)^, Pointer(s)^, Length(s));
 ШК.Text := s;
end;