Well, if your scales are correct which it shows to be with a Jefferson at 5 grams, then you may very well have a slightly thin planchet or either enough metal has been worn off of the Quarter to reduce its weight. I don't recall the weight tolerances of a Washington but over a gram is still quite a bit which I know is not within Mint tolerances... As far as the diameter, your coin has been worn down which is very obvious as evidenced by the worn reeding. It may have spent some time in a clothes dryer AKA dryer coin
or could just be a result of circulation which I am more inclined to believe by its look. We are only talking a few tenths of a millimeter, if that, in diameter difference which would be totally normal for a heavily circulated Quarter. I would not consider the diameter to be an error at all. Thin planchet..... possibly so. I say this due to its approx. 1.2 or 1.3 gram weight difference. Hard to tell about the strike due to the circulation damage, wear and tear. Just MOH.