The Cadbury Brothers introduced their fondant-filled chocolate egg in 1923, which now sells some 200 million per year. They assure us that it has not got smaller over the years - it is just that we've got bigger.
Imitation fondant eggs are not entirely new. See the 15th Century
Creme Eggs for Lent

Cadbury Creme Egg, whole and split
Image: Evan-Amos