Riddle is of course the goldfish in her double speak. It's unclear if she knows about his history, or is she's just speaking to what she literally sees. Riddle is a smaller, stunted kid, kept in captivity, but when released into the wild, Georgina sees Goldfish grow in size, beyond what was thought possible of them.
It could be in reference to his smaller stature, which was likely stunted by his extreme dieting, or even his perspective, as he's famously narrow-minded and sheltered.
Georgina seeing Riddle's potential mirrors not only Floyd's desire to confront him honestly, but his own mother's desire to push him past the limits of what's "normal" for his ability and age*. But rather than push him to succeed by her narrow standard of success, Georgina implies that Riddle can be much bigger than what's expected of him. That he can literally outgrow the confines of his captivity, or the life that's imagined for him.
**Book 6 Idia analyzes the overblotters magical ability and concludes that, while his capacity for magic is average, Riddle's ability is inflated by his training.
