how to measure kezayis/kebeitza of bread and cake

For brachah achrona, birchas hamazon and washing, how does one measure the volume of bread or cake? Is it the volume only of the flour (which would mean that a kezayis of cake for brachah achrona would potentially be a much larger piece than a kezayis of bread for birchas hamazon)? How would you figure out that volume? Also, whole wheat flour is made by first making white flour (separating the bran and the germ from the endosperm) and then later adding the bran and germ back into the white flour (in stoneground flour, however, the whole grain is ground and components are never separated). Does that the components of the flour are separated and then recombined have an impact on the measurements of volume? What about when it comes to taking challah?

We measure the mixture of the dough after its baked by volume. The only things that wouldn’t be allowed to be measured together with the dough are extra things like pieces of fruit, chocolate chips and the like. Same with Chalah.