Hm, on the one hand, I'd translate it as "tall", too, at first thought, but on second, my half forgotten school French seems to tell me it can indeed be meant as a reference to her weight rather than her height. Either way, the context makes it clear it's a dissing.
Re: G1 state visit as per Wilhelmine
He embraced me, and, turning to the queen, he observed : “Your daughter is very tall for her age.”
(In Der Thronfolger, he says "plump for her age". I would like to know the French original word here.)
Per Gutenberg:
votre fille est bien grande pour son âge.
Re: G1 state visit as per Wilhelmine