
Copper flattens out easily under normal circulating conditions due to its relative softness. I once did run across another reference to "large 0" and "small 0" varieties of the 1940 penny, but I've searched hundreds of 1940 pennies since then and haven't found anything conclusive. I think the trick to finding any differences in sizing of the date or inscription is to compare higher grade coins, which don't exhibit any great amount of wear. Your "thin date" coin seems alright, so keep your eyes open CPC, and you may yet find that true "fat date" penny.