One headcanon I've got is that there are different subspecies of Seadragonus Giganticus Maximus: sea dragons start off small, but their growth and lifespan seems to be non-linear and almost unique to each individual. Furious, Toothless and even possibly Wodensfang aged differently throughout their lifetimes, with Furious reaching his prime much earlier than Toothless. I think we can rule out malnutrition since Hiccup definitely pampers Toothless, and it's likely much harder to hunt for food in the wild in Furious' case.
It's possible Furious is a subspecies of Seadragon that become much more imposing and powerful early on in their lives. I imagine that subspecies is tends to travel a lot in search of food, and may be why Furious ended up being adopted by the Grimler dragons as a baby.
Toothless (and possibly, Wodensfang) belong to another subspecies of Seadragons. At the start of their lives, they appear to be very similar to Common or Garden Dragons as a defence mechanism. Some organisms have adopted mimicry to deceive predators. and Considering younger Seadragons are at their most vulnerable, it makes sense they'd disguise themselves as non-threatening species to survive (Emsleyan mimicry). In Toothless' case, he ends up living with a colony of dragons.
It's possible Seadragons are like orcas; different colonies have vastly different diets (some prefer salmon, and others prefer seals and so on). These factors may influence their physical growth. I think these two subspecies of Seadragons are similar, being exceptionally intelligent apex predators.