How is the small side of a lamb the "large" side?

Question

Maseches Tamid, chapter 4, describes how the lamb offered each morning as a korban tamid was butchered. It says:

He arrived at the left side.… The spine was with it and the spleen was suspended from it. It was large, but they called the right [side] "large" because the liver was suspended from it.

Note that the Rosh clarifies that the left side was larger because it included the spine.

Why did the liver made them call the small (right) side "large"?

I've found two answers to my question above. However, I'm unsatisfied with each of them, as I'll explain. I seek alternatives to the following answers and/or solutions to the problems I have with them.

1. The commentary printed with the Bavli explains that "the left side was heavier [than the right], but they called the right [side] 'large' because the liver was suspended from it and the liver is not part of the side. Since something not part of the side was suspended from it, it was called 'large'."

   Makes sense to me, but: Is the spleen a part of the side more than the liver is a part of the side? (I can't find good pictures online.) If not, how do we understand this commentary?

2. The Raavad explains that the right side was in fact heavier, because the liver was heavier than the spine and spleen combined.

   But: What, then, does the mishna mean when it says the left side was large?

Answer 1

Art Scroll Gemoro Maseches Tamid (Mishnah) 31a3 note 31 explains that the Raavad is referring to weight. However, the left side was "large" refers to the size of that side because of the spine extending out. Thus, had the mishnah not said it explicitly, then the Kohanim would make the mistake of thinking that the left side (being larger) was the one that was supposed to be picked up in the right hand and could drop the side with the liver.

Note also the picture of the lamb on the bottom of page 31a3 which shows the lines of the various portions. The picture shows that the right flank is greater in volume as it includes the top part of the left side but the spine itself is part of the left flank (though not the meat on top of it). This causes the left flank to appear greater even though the spine itself is light.

This note says

Raavad questions what practical difference it would make whether the right or left flank is called the "greater one". Raavad suggests that "the greater one" means the heavier one: An unsuspecting Kohen may not realize that the right flank is heavier and may attempt to take it in his [weaker] left hand. This could lead him to drop the sacrificial limb. Raavad adds [within this answer] that the right flank should be held in the right hand for another reason too: because it is the more distinguished limb. Thus, the Sages called the right flank the greater one to alert Kohanim that it should be held in the right hand (cf. second explanation)

Rav Kahati seems to say "because of the kaved" which implies because of the liver however, this would not be clear whether he means the first or second explanation that you give. Ikar Tosfos Yom Tov and Tiferes Yisrael on the Mishnah seem to say that it is because the liver is separate organ and not part of the "space" of the dofen (flank) but attached to it, then it is called "big". (Your first explanation) The answer to your question about the spleen, seems to be that the liver is a more important and bigger organ so that even though the spleen is on the left side (as the mishna says "suspended within it") the liver would override it in importance and cause its side to be called gadol.