Awww, she kind of looks like my Linney but with a crest on her head! Honestly, I think she is likely just fine. Being overweight is never a good goal, health-wise. She may just be on the smaller side naturally (I had one guinea pig who never hit the 2-lb mark in her life, ever! She was fine... she was just tiny, very dainty little paws, small bone structure in her face and head, etc.) I would just focus on feeding her healthy foods with good variety and let her grow at her own pace.