The gemara raises a concept of an item which is batul whether it is viewed as כמאן דאיתיה דמי. The gemara assumes that if we regard an item that is batul as if it doesn't exist then when a tamei item is batul it wouldn't even be me'tamei by carrying, but if we regard it as existing even while it is ba'tul it would be me'tamei through carrying but not through touching. The gemara leans toward the distinction between carrying and touching implying that we view the tu'mah to be in existence even while it is batul. The rationale for the distinction between carrying and touching is that when one touches something they cannot be touching every molecule at once. Therefore, we can only consider him to be touching an item of tu'mah if the majority is ta'mei, otherwise we would consider the entity to be not tamei. However, when it comes to ma'sah, since when something is carried, every molecule in it is carried, the existence of the tu'mah even as a minority would render the carrier tamei.
Tosafos (d.h. 23a) asks that eating an item should be more similar to tu'mas ma'sah than to tu'mas ma'ga. Why do we permit one to eat an issur that has been batul in rov? Since the minority is considered to still be in existence, it should be comparable to ma'sah where the entire item is being carried or eaten and therefore should be eating issur? Tosafos answers that when one eats something, every small amount that is swallowed is batul b'rov and is therefore similar to ma'ga where the touching of multiple times doesn't make it as if were touched at once. Therefore, just as by tu'mas ma'ga we would identify the entity based on the majority ingredient, the same would be for eating. Tosafos assumes that it would be impossible to eat the entire amount all at once. Perhaps it is because the בית הבליעה can only contain a beitza at one time which it 2 kezaysim. For there to be a full kezayis of issur in the בית הבליעה at once of something that has been batul b'rov, there would have to be more than 2 kezaysim in the beis ha'blia at one time, which is impossible. Furthermore, there should be an issur of eating even a chatzi shiur of issur, unlike tu'mah for which there is no contamination if it is less than the amount needed to be metamei.
The Rosh (Gid HaNashe end of 37) asks the very same question as Tosafos except that the Rosh preempts the answer of Tosafos in his question and says that one should have to be careful to eat in a way where every swallow could potentially contain only the heter food and not the issur food so that it would be more similar to ma'ga than ma'sah? The Rosh answers that we cannot compare tu'mah to issur. There are two types of tu'mah, therefore the chiddush of bitul b'rov may apply to one and not the other, such as to ma'gah and not ma'sah. However, regarding eating, if we would consider there to be any issur in what the person was consuming, the Torah wouldn't allow it's consumption. The fact that the Torah allows one to eat issur that has been ba'tul is a gezeiras hakasuv that in the world of issur, we view it כמאן דליתיה דמי, as if it weren't in existence. Therefore, a piece of issur that is batul in two pieces of heter can even be cooked together (on a torah level) and eaten all together as if the issur disappeared.
The approach of the Rosh seems to assume that the concept of tu'mah that has been batul being metamei by carrying to be d'oraysa, whereas Tosasfos concludes that it is likely only m'drabonon.