Fridge Logic: Gwendolyn was capable of fitting into her mother's dress, indicating that's about as big as she's going to get. But just LOOK at King Odin. Hot Skittyon Wailord Action anyone?
Just to play with this line of thought, no one said it was the last size that her mother ever wore — maybe she outgrew it before she died.
Odin is capable of shapeshifting. You don't think he was that size when he infiltrated Titania, or when he secretly seduced Velvet and Ingway's mother do you?
Otherwise you could interpret his size as being partially metaphorical for the amount of power he wields.
In general it seems that Aesir/Berserkers grow bigger with power and age, but this doesn't apply to Valkyries (who actually lose power when married): So probably, yes, Gwendolyn's mother was that small compared to Odin, but is probable that Gwen and Griselda were conceived when Odin was less powerful (and thus, smaller) or that Odin shapeshift in a smaller form in order to have sex with her.
Leventhan is the last dragon born in Erion. In that sense, he's really the last one.
Onyx and his subjects need fire to survive, and Ringford is full of wood to burn. Still, his smug ignoring and disbelief of the prophecies was pretty damn stupid of him.
