Sometimes it happens that a breeder may lose a baby from a litter that is smaller and just not as strong enough as the others. Thats where I step in and I feed the baby and help out. It happens and its not saying that baby is "sick". there can be many reasons.
As far as her size and growing. I have found that as they mature they may not get "bigger" but will gain weight and "fill out" up until they are 2. This may just be her size and how she is going to be. Genetics has a way of fooling us. Size of mom and dad don't always play a part. She can be picking up a gene from either parents side way back.
She's a cutie! Not all small dogs are "sick" or have health issues
My male will be 3 in Dec and he is only 3 lbs. Healthy as can be