The gluten proteins are so diluted that they cannot form the structure. Instead, egg proteins are critical. This can be illustrated by reducing the flour slightly. Little change occurs in the popovers. When the same weight of egg white is substituted for the whole egg, there is also little change.
Egg whites have a great ability to expand and give volume.
However, with the same weight of egg yolk, the low volume shows that egg yolks are less elastic than the egg whites.