I wonder whether the fact that the Hazelnuts are not getting eaten is why they are bigger this year ?
I can't comment for your area but we've had a fairly cool and dry year down here. I think the large hazelnut crop here is down to the fact we've not had a hard late frost, of much frost at all.
The plants I collect from produce large nuts every year, I'm sure they were more productive a few years ago but they are getting shaded out a bit by other trees in my woodland.