|MadSci Network: Genetics|
To the best of my knowledge, there has never been a study of what might cause especially large ears. In my own experience, I have never noticed that people (presumably you mean Americans) several decades ago had any significant difference in ear size from the present day average. If your observation is correct, it is unlikely that the difference is due to genetic variation. When such morphological traits change through genetic alteration, it takes a very long time for this variation to appear - certainly longer than 40 years or one generation! Such traits are likely controlled by more that one gene, so multiple genes would have to be affected in order for such morphological change to occur. Additionally, the gene pool that you see in the pages of your parents' yearbooks is largely the same gene pool you see in your yearbooks, with just a single generation's genetic change separating the two. If your observation is correct, it would be more likely due to environmental changes, which include a bewildering array of possibilities. I might point out that your observation may not be due to any genetic or drastic environmental fluxuations, rather it may be due to a different kind of variation - different styles of haircuts yield different degrees of ear exposure. It might just be that haircuts 40 years ago were much shorter on average, making ears look larger when compared with the more concealing coiffures of contemporary culture.
Try the links in the MadSci Library for more information on Genetics.