Remek, I still don’t think it has to do with HOW MUCH blood you have. Your body can reduce or increase flow to the body parts that need it. For example, your cardiac output can increase 500% while stenuously exercising compared to at rest. When your muscles need oxygen, your heart rate increases to bring more blood to them.
The girth gains related to erection quality probably has to do with tissue healing as I theorized earlier, but I have no idea why giving blood would effect your erection, unless is it psychosomatic.