Well, think about it...Eragon turned into a semi-elf in Eldest thanks to a dragon's miracle. Is it not plausible that the writer of the script thought likewise? "The situation is dire, so we'll have Saphira grow!!!" I think they could've expanded on it. Perhaps it could've been that the dragon sensed the Empire could come at any time and thus just preformed her first "miracle" out of nowhere. Brom said a dragon's miracles were usually unpredictable and random even to dragons. I don't think little Saphira had any clue that she was going to grow so fast...until she landed and looked herself over briefly.

I've read the book, thank you very much, hehe. Brom mentioned dragon miracles being random and unknown. Dragons don't even know how they perform them. I suggested that the writer of the script (different from Paolini) perhaps thought a "dragon miracle of super growth" would be a great way to save time.