not more matter, the milk’s molecules are converted to gas molecules by chemical reaction, prob part of the bacteria growth/fermentation process, gas molecules want to be farther away from eachother than they did as liquids, and this was definitely the case here, because the molecules once converted to gas create pressure inside the bottle, the more milk molecules that the bacteria eats, the more gas is created, and the more pressure is built up inside the bottle until the seal breaks.