Can't upvote this enough. In fact I was about to write this myself. Realism, as per usual, is silly to expect in a high fantasy setting, where sword wielding fighters can summon magical swords from the sky.
Credibility is the key word. For instance, it was even said in the opening post: "Unless dragon genes make you stronger than you look..." Dragons are creatures that don't exist, and since Shyv's humanoid form is clearly an act of shapeshifting, we can easily buy into her being stronger than her humanoid form would suggest.
But the others are (largely) human and from everything we can observe, the body shapes of humans in LoL reflect closely what we know of real human bodies, so the lack of any other explanation makes their massive strength look strange, when compared to their stature.
Another example would be the grrl-power webcomic. There, superpowered women all have rather ahem "outstanding" breasts, despite some of them being visibly muscular. The comic addresses that by saying that yes, that this strikes them as odd and that according to their observations, EVERY super (male or female) looks almost perfectly ideal, but since they don't know where those superpowers come from, they just guess it is an accompanying trait.