The muffin started out as a small flat griddle cake and was eaten as a substitute to cornbread. The modern muffin is sweeter, looks like a cupcake and is has many other additions made to it. It was widely popular as a breakfast item because it was quick to assemble. Due to the fact that they grew stale quickly they were not marketed as a baked good until the middle of the 20th century. Essentially a muffin was a type of bread that was baked in small portions for quick, portable eating. Now it resembles a cupcake and had much of the same ingredients.
Early muffins were limited to certain ingredients and additives like nuts and dried fruits. The earlier muffins were English muffins, flat breads that are buttered and eaten with tea. The American muffin is cupcake like but still pays homage to its English ancestor in that it is not as sweet as a cupcake nor are they that big in size.
Another difference between the English and American muffins is that the English one is yeast leavened and the American one uses a chemical leavening method (baking soda, baking powder) to raise them. The English muffin was cooked in a disc shaped tin with flat sides, about 8 cm in diameter. American muffins began to be made in cupcake tins but with smaller molds. With the advent of paper baking cups the iron tins eventually took a back seat. Nonstick muffin pans have allowed this baked good to be made in a variety of shapes for holidays and events but the traditional circle shape, small enough to fit in an adult palm.
The majority of the changes described above came in the 1970s and 1980s. This was mainly due to the fact that more women were entering the workplace and there was a decline in home baked goods. This along with the rise of smart living and healthy eating demanded that the traditional muffin recipe be reformatted. Preservatives were added to commercial mixes to make them sell. But using healthy ingredients like grains and yogurt required a higher amount of fat and sugar to be put in order to make them last long enough to be sold to the public. This is when the traditional muffin took on the shape of a cupcake in reference to the amount of calories they contained. Hence, the humble muffin became the larger mammoth sized version that we know today.