Rashi on the top of 30b explains that the "פחות מכביצה" of dough that attaches the tamei dough with the tahor dough is removed immediately with the krias Hashem. Presumably, rashi is writing this to explain why the tahor dough is considered chulin and not teruma, since it will be turning into challah during the process. However, at this point rashi is explaining the braisa of where the dough attaching is less that a beitza and is therefore too small to be metamei anything else, so why would it have to be removed at the time of krias hashem? Is rashi just saying this to explain the braisa that says the attaching dough is a beitzah, in which case it would clearly have to be removed at the time of krias hashem so that it is not metamei the tahor dough that was just turned into challah? Rashi himself (d.h. mai lav) says that the advantage of using less than a beitzah is that you won't have to be so careful to break that attachment at the time of krias hashem.