From: Rabbi Moshe Sofer
To: Rabbi Elyakim Getz Konigsberg
Rabbi Getz asks whether esrogim measuring only the size of a contemporary egg are invalid, given the Tzlach's thesis that eggs have shrunk. The Chasam Sofer rules they are valid, arguing that all natural objects have shrunk proportionally. He concludes with his famous dictum that novel stringencies unknown to our forefathers should not be imposed.
Source: Shu"t Chasam Sofer OC §181