tiny acorns

Forgot I had taken this photo a few weeks ago, at the tail end of collecting acorns out of the yard. The ordinary majority are about the size of my fingernail, and sometimes I find ones half sized- just as wide, only flatter:

But this year I found quite a few very tiny ones as well. Perfectly round, and little neat caps, but very small. I am not sure if there have always been tiny acorns and I just never noticed them before, or if some are underdeveloped because the trees were stressed? We had a very dry, hot summer. Small ones-