How was Shuriken able to grow bigger than Gaeldr if he was born after Gaeldr?
Because Galbatorix used magic to make him bigger
Great swaths of stars vanished as Shruikan wheeled across the sky, climbing higher and higher. Eragon tried to guess the dragon’s size from the outline of his shape, but the night was too dark and the distance too hard to determine. Whatever Shruikan’s exact proportions, he was frighteningly large. At only a century of age, he ought to have been smaller than he was, but Galbatorix seemed to have accelerated his growth, even as he had Thorn’s.
Inheritance - Fire in the Night
(emphasis mine; no chapter number)