var scale:Number = THUMBWIDTH / IMAGEWIDTH; // or THUMBHEIGHT / IMAGEHEIGHT, assume nalang na THUMBWIDTH / IMAGEWIDTH = THUMBHEIGHT / IMAGEHEIGHT

var matrix:Matrix = new Matrix();
matrix.scale(scale, scale);

var smallBMD:BitmapData = new BitmapData(bigBMD.width * scale, bigBMD.height * scale, true, 0x000000);
smallBMD.draw(bigBMD, matrix, null, null, null, true);

var bitmap:Bitmap = new Bitmap(smallBMD, PixelSnapping.NEVER, true);