It is rare to see bloating in porcelain because of the purity level of kaolin. However, some porcelain bodies do incorporate ball clay/s as a plasticizer. However, in this case I have reason to think that potassium was used as a flux in lieu of sodium. I have seen this kind of bloating when experimenting with porcelain bodies that incorporated mica and pearl ash (pure potassium/s) in too high of a level. (over-fluxed)
In your first picture: the bottom of the piece is pure white on a pure white body. As you move up the side wall, you can see a shift to a tan'ish color underneath the white glaze: a classic sign of potassium fluxes. Potassium is the off-gassing hog of the KNaO group; and it will discolor a porcelain body; if it is over fluxed.
The bottom of this test bar is pure white (vitrified), and it turns the tan color as you move up the bar: as seen in your sidewalls. As potassium off gases, most of the gas remains inside of the body: only about 3-5% is off gassed into the kiln atmosphere. When potters turn their pieces upside down and fire on the rim: they assume the gas is trapped inside- not true. Sublimation requires a complex answer, so I will leave it at that. So I would put potassium (too much) as the primary cause at this point.
Firing in a semi filled kiln only effects rate of climb and rate of cool.