Quite often a jpeg image blob filesize will grow when reading back the image blob data!
eg.
$img = new Imagick();
$img->readImage($filename);
$sizeA = $img->getImageLength();
$blobdata = $img->getImageBlob();
$sizeB = $img->getImageLength();
Quite often sizeB will be greater than sizeA depending on the jpeg image being read