She
is chubby, but she's also extremely tall. Toddlers usually
are chubby, that's just how they're built. I think she's built just fine, she's just unusually tall so people don't think of her as being as young as she actually is. To find out if she's out of whack for her age/height, you'd have to take into account the typical sizing for a toddler, then try to bump that up to match her height, then see how the weight fits in with the adjusted height.
My niece was chubby until maybe a year or two ago. Now I think she's just a little chubby, partly because she's finally grown into her baby fat, but also because I think her mother has a thing about weight and is worried about her daughter's. (But not her son's, my nephew is also tall for his age (which is 10), but he also wears a size 16 Husky!)