The gemara suggests that perhaps the halacha of following rov only applies to רובא דאיתא קמן, a rov that is in front of us such as the majority of the sanhedrin voting on an issue, or 9 kosher stores and one non-kosher. But a רובא דליתיה קמן may not command the same status and we wouldn't follow it. The gemara rejects this from the fact that we follow the majority in assuming that most children will not turn out to be a s'ris or ay'lonis, even though that is dependent on a statistic of רוב קטנים לאו סריסים ורוב קטנות לאו איילונית נינהו, which is a רובא דליתיה קמן. However, why do we consider the rov by ketanim to be a רובא דליתיה קמן - just as we consider the 9 stores to be a רובא דאיתיה קמן, we should consider the rov of ketanim to be רובא דאיתיה קמן since most ketanim that are alive in the world today aren't s'risim or ay'lonis? Rabbeinu Gershom seems to address this question and writes- דלא חזינא רובא דעלמא אי הוי סריסים אי איילונית אי לא ואפ"ה אזלי בתר רובא. Even though we have a statistic telling us something that exists right now, since the numbers aren't obvious and can't be counted in front of us, it would qualify as a רובא דליתיה קמן. Similarly, the fact that most cows aren't treifos is a רובא דליתיה קמן because the statistic will not create a רובא דאיתיה קמן since they cannot be counted in front of us.
On another note, the gemara says that we don't follow a rov that is dependent on an action such as the fact that most animals would be pregnant by a certain age, which is dependent on the action of mating with a male. The rationale for not following this rov, the Ramban (Halachos of Bechoros) writes that a mi'ut which happens by itself is stronger than a rov that is dependent on an action. Rashba in Chulin 9a says that this limitation is only d'rabonon. Meaning, m'doraysa we follow rov even if dependent on an action but the Rabbonon tell us not to. Therefore, the Chazon Ish writes that we can only be machmir to not follow a rov dependent on an action, but we cannot be lenient to do so. The Binas Adam (sha'ar rov v'chazaka) proves that the Rashba is correct that it is only d'rabonon based on Tosafos ד"ה אי בעית אימא who says that R. Meir who is concerned for the mi'ut is only d'rabonon. Since the first answer of Rava that the concern of the mishna that the animal born now is a bechor is only m'drabonon, the answer of Ravina about not relying on rov which is dependent on an action is also only d'rabonon. However, this proof isn't very solid because when there is a chazaka that supports the mi'ut even R. Meir would agree that we are concerned for the mi'ut even m'doraysa and here there is a chazaka that supports the mi'ut (as tosafos explains in d.h. me'chvarta). The issue of whether R. Meir is concerned for a mi'ut only m'drabonon or even m'doraysa (when there is no chazaka), is a dispute between tosafos and mordechai (mentioned in this blog on the first perek of chulin).