1) Actually, the Gemara asked that only the Bechor should be forced to do Yibum , not Chalitzah.

2) The Rashba explains the Gemara's answer as follows. It answers by rejoining that the questioner said that when there is a Bechor, the Bechor is forced to do Yibum. What, though, is the underlying reason behind the fact that the Bechor has to do it? It must be because the Bechor is the oldest, as the fact that he is a Bechor does not really make any difference. Since he is the oldest it follows that this is the reason why he is expected to ensure that the name of his brother is perpetuated. If it all depends on age, then logically it follows that even if there is no Bechor, the oldest brother should do Yibum.

3) The Rashba effectively is saying that it does not depend on being a Bechor at all, but it depends solely on being the oldest.

4) The Tosfos ha'Rosh also states the same idea clearly. He writes (end of 24a, DH k'Bechor), "What is the reason why the Torah stated that the Bechor should do Yibum? -Because he is the oldest. Therefore, it makes sense that if there is no Bechor, or if the Bechor does not want to do Yibum, the most important way of doing the Mitzvah is through the oldest brother."

5) The Ritva writes a similar idea but also explains how this fits with the word "Bechor," which is the word actually used by the Torah. He writes that the comparison of the Gemara between Bechor and the oldest brother does not originate from the word "Bechor" used by the Torah. On the contrary, if we would follow the literal meaning of the Torah, we would require that an actual Bechor do Yibum. The Ritva explains that the Gemara is using Sevara, logic, and not the literal meaning. Logic tells us that just as when there is a Bechor, he does the Mitzvah since he is the Bechor, so, too, when there is an oldest brother, he does the Mitzvah because he is the oldest. This is because the fact that he is the Bechor is not the crucial point at all in Yibum. The only reason why the Torah mentions "Bechor" is that he is automatically always the oldest.

6) These three Rishonim are all saying the same thing, just using slightly different words. What it comes down to is that when the Torah says "Bechor," it does not actually need to be the Bechor; rather, the word "Bechor" is used to indicate that it must be the oldest brother.

7) To sumamrize, when the Gemara compares the Bechor to the oldest it is really saying that the reason the Torah uses the word "Bechor" is that he is the oldest.

Kol Tuv,

Dovid Bloom