Encoding from existing JPG to JPG significantly increases file size

[ToshY]

I've just noticed that using the encode method increases the file size significantly if you try to encode a JPG to JPG.

Example:

$image = $this->image->make($originalJpgImage);
$imageJpgEncoded = (string)$image->encode('jpg',100);

Results with GD:

• Original: 42 KB
• Encoded: 112 KB

Results with Imagick:

• Original: 42 KB
• Encoded: 61 KB

Setting the quality 80 gets me decently close to the original file size, but I fail to understand what is causing such an increase. Could someone elaborate on why this is happening? And is this normal behavior?