Its not tight. The lips arent pinched inwards and the corners are free to expand. The horse has a low set facial crest (facial bone) lower than the horse pictured in Smrobs pictures (ie smrobs second picture is a good picture to look at.) and a short muzzle. Your horse is also wide in the facial area beginning from the lip corners going backwards. This is common in young still developing stock and in QHs. Having a shorter purchase like the one pictured (your picture) will definitely help with this problem. The purchase should not dig into the face or rub to the point of causing harm or discomfort. Resting lightly against the face with out impedeing upon it should not be cause of alarm. The mouth piece of the bit seems to be of correct size....no puckering at the corners and not pinching and leaves just enough room to allow the corners to expand a little. Its resting in the mouth nicely not to tight and not to loose. I like one wrinkle in the corner of the lip (esp with snaffles not so much with curbs) but with some horses (depending on the individual) being snug into the corners of mouth with out a wrinkle will work just fine also. If you choose to go wider I would not go any wider than 5 1/4 inches (mouthpiece). Opening up the purchases a little more may be beneficial if you wish but it does not appaear to be digging into the face.
"The question is not, can they reason? nor, can they talk? but, can they suffer?" Jeremy Bentham