Never seen a slim bird call herself 'bubbly', it's generally accepted as a term for overweight.
I see it on slim ladies adultwork profiles all the time. It's really nothing to do with them being overweight.
"(of a person) full of cheerful high spirits.
"a bright and bubbly personality"
synonyms: vivacious, animated, ebullient, lively, full of life, spirited, high-spirited, scintillating, vibrant, zestful, energetic, dynamic;"