@1000sunny
Yeah, I generally agree with you there, and yes, I've read those too so I definitely see your point with those examples. I certainly think that is true, that his immature body is likely a huge limiting factor in him releasing his 'full power.'
Though I do think there is a very slim possibility that the difference between his 'forms' wasn't just his size, but I think you basically mean about the same way I'm thinking about it; so this may be just me being... 'too literal' or whatever.
I mean if the usable power he had was the same in both forms, the smaller form could be considered 'stronger' due to it being more concentrated. Kinda similar to using high pressure water compared to using the same amount of water, but over a longer time or larger surface. But in this case his smaller form may indeed 'have' the same reservoir of power, but as you said, his immature body can't handle it or is otherwise limited from utilizing more of his 'full power.'
Like how humans are thought to have a 'limiter', of a kind, so they generally don't use their 'full strength,' which could cause themselves damage in various ways--torn tendons/ligaments and/or broken bones for example. Or how people under the influence of certain substances (pcp is a good example) can often over-do something and injure themselves or others.
One other interesting example of this idea that I've heard about irl is found in hand to hand combat, though I only saw a couple academic papers on the subject, and I'm far from an expert. Anyway, in boxing they use the padded gloves, which many may think could help prevent damage to the opponent, but in reality it's more of protection for their fists, which often leads to more brain damage (or whatever is hit) than compared to when using bare fists, because they 'know' (consciously or not) that they can put more force into it without just shattering their hands quite so easily.
Having said that, if the monster ranking is based on the possible power they can wield, at a given time, then I still think his rank at 'birth' may have been lower than S, but yes still certainly on the higher end. If this wasn't the case I would think the limits would have been (probably temporarily, and likely leading to some damage, but damage is better than death) 'broken' when he was first injured so badly. Though this could also be more due to him not being accustomed to his new body, and maybe the power was indeed there, but he just didn't know how to properly use it or something like that.
But even in that case, if they have the power, yet can't use that power, for any reason, should that actually count towards one's rank? I wouldn't think so in this case as the rank doesn't seem to include their potential, but just the danger/strength at that time. But I also suppose adjustment of a rank depending on temporary conditions could get... annoying and overly complicated, depending on where the 'line' is drawn.
Aside from all that, when he changed form, I agree, he was likely just more easily able to use his 'true power' at a higher amount, though I could see it being a bit more than that too, but I don't have any 'proof' of this supposition. I mean, say he could only use 30% as lil Tama, and larger Tama can use 100%, but how are buffs or the like counted? If not included and he was buffed to be able to use 120% for a short period, should that count towards ranking? I lean towards that it should be included as it can certainly increase the possible damage, but if it is a one time thing, say from an item or another's spell, then it isn't a 'common risk' and could bump the rank up to a misleading level.
I don't think that part really mattered at that point anyway, when in his larger form iirc, I think everything he did was using his own power, which could be done again, though again possibly injuring his still immature body; so I would think that should go towards his ranking. I just kinda mentioned it as a possibility, though I don't think it actually applied in this case.
This would of course change though if his larger form also provided additional 'boosts' that his smaller form just wouldn't have 'access' to. Kinda like how evolution can work in a Pokémon game. You can keep them in their early form and just continue to lvl up without evolving and they will indeed keep getting stronger, but by evolving into the next generation their stats, and the growth rate seen at each level increase, can be changed. It's been quite a while so I may be wrong, but in the case of Pokémon games I think these changes can be both good and bad, with some stats increasing more and some less in their other stages; like after evolution maybe speed increases at a higher rate, but maybe defense is now growing slower, at each increased level. Ah, and yeah, it probably isn't gonna be exactly applicable to this series, just an example I thought would be simple and well known.
Oh and just in case it's coming across wrong, I'm not meaning to be argumentative or anything like that, I just enjoy this kind of discussion, so 🍻. Especially enjoyable when going more 'in-depth' than one may have originally thought about a topic, and getting another's perspective on it. It can be much easier to find any misunderstandings or blinds spots (or whatever) thanks to this.
So, yeah, generally I agree with you, though I think it may be... more complicated. Oh and I know I could be totally off on this, and proven wrong at the author's whim in following chapters, or on a detail I missed/forgot (or translated in a way I misconstrued, etc.) from previous chapters.
Cheers, mate.