I have a similar issue. My male albino is 852g and a bit over two years old He's gained 40g in the past three months. He's only a few inches over 3 feet and has a healthy thickness to him. His previous owner raised him on mice, but I think he was only fed 2-3 of them every other week. The vet said she was surprised at how small he was for a two year old, but proportionately he's fine.