I'd say that was average, my son was 7lb 10oz and he was born 2 weeks early, however he is a chunk now, he's the same age as mabel but eats like a horse and is wearing 2-3 years clothes, where mabel is still in 6-9/9-12months.Definitely an obsession with Mabel being small and petite. Exactly the same as Sarah Ingham from the Ingham Family channel who sleeps her almost 2 year old son in 6-9 months!! Mabel was 8lb 4oz at birth which is average if not more than.
Personally I think its all down to Mia not giving her what she needs, no nutrients, vitamins, stuck in a room, no love at all, and I've always been told that if your child is neglected, they aren't going to grow to their full potential, where a healthy, loved baby will.