I feel obligated to add an extra two cents after leaving an unintended impression. Though I wish it were the case, I am not the sole authority on matters of Scar
Others have rights! This is a restatement of their ideas and how I see the relationships between them.
Paraphrase of Question: Why did Scar, being younger than Mufasa and closely related (a brother) to Mufasa (a seemingly well-nourished, strong lion), and having the same *available* dietary nutrition, appear physically weaker, thinner, and older than Mufasa?
Summary of Opinions: All are agreed as we join the stampede/You should never take more than you give--in the circle of life! Oh wait, no, that's not what I was going to say. What I meant was: Everyone's opinion on this debatable matter is equally valid, since there is no *known* explicit explanation from an official source (except the ones mentioned). Also, all agree that the assumption (Scar being weaker and appearing older) is true.
-Since The Lion King incorporates many patterns of the real world into its fictional story [features animals of existent species whose behaviors and physiologies are able to be studied, features some events/ideas that could happen in reality (birth, death, male lions having manes, lions climbing rocks, e.t.c.), features a real continent on a real planet (Earth)], it is meant to reflect reality to a certain extent. The behaviors of the animal characters are supposed to match their respective species to a certain degree. Example: The Lions eat meat (Simba eats zebra/the lions eat antelope). They live in a pride composed mostly of female lionesses, but ruled by a single male (usually there's not more than one male in a lion pride, and this is reflected by the movies' power struggles). The male lions gain dominance through physical fights. This is a valid point.
-Since The Lion King also includes many highly unlikely events/ideas, we can definitely assume that its reflection of reality is between zero and one, but not equal to either. Where its "loyalty" to reality lies--between zero and one--and thus how its unexplained events and ideas can be explained--is a matter of opinion. *Even whether these unexplained events can or should be explained is a matter of opinion, though one of the major goals of this forum is to explain the unexplained. However, despite the goal of this forum, in my opinion, an explanation that is not undeniable, even if it
fits the evidence perfectly well, ought not to take precedence over no explanation, as long as there are alternative explanations.* Some lean more towards realism in their opinion; others lean more towards non-realism. Some examples of highly unlikely events/ideas that The Lion King includes are: talking animals, a warthog and meerkat living together and "raising" an orphaned lion, a lion carrying out premeditated murder, a meerkat not only standing, but walking, on his hind legs, e.t.c.
On The Plot Devices/Artistic License (Distortion of Reality) Side:
Some say the cause of the question's assumption is a plot device. Because of Scar's weakness, he is forced to find a "clever/unconvential" alternative to a physical challenge of Mufasa's authority in order to replace Mufasa. (Si Targaryen, DGFone). This alternative also benefits Scar by not provoking suspicion in the lion pride. What I call "artistic license" is what PrincessKiara, I, Zulama, and Todd23 underlined; it is Disney's employment of symbolism/associations/and such in TLK's characterizations.
On The "Following Nature's Patterns" Side:
Annie's idea was that such a difference (as the one between Scar and Mufasa) between closely related individuals of a species is definitely possible, since she has observed it herself. Julie added that this is not only possible, but probable in nature (this is true). Some attribute the cause to Scar's diet, whether that attribution is where it is obtained (SimbaWinter) or whether Scar will willingly obtain it (TheBlackCatCrossing). I myself like the idea that Scar was too proud to take Mufasa's offers as food, but this is just speculation. Scar's quote about "the shallow end of the gene pool" validates the genetics argument for sure, not to say enviornmental factors (like being a runt) couldn't have added to Scar's misfortune.
Mixture of Both Sides:
Scar, because he was born physically inferior to Mufasa, due to genetics or environmental factors, becomes jealous because of Mufasa's physical superiority, fueling his evil motivations (Todd23). (Lions can't be jealous, but every organism responds to its enviornment and exhibits heredity.)
I personally affiliate with the "minimal involvement of natural patterns idea," as to me, a mixture of moral capacity in animals with the cruelties of nature is just one bad combo. I like the thought of Scar's killing and gladiator matches being an exception to the rule for the lion society, not the norm. I also like the idea of animals of different species (like Timon, Pumbaa, Simba, Rafiki, and Zazu) being friends, which isn't often seen in nature.
<-- Unlike Scar, I concede, you guys are entitled too