It's absolutely not sustainable!! These are known as "Newby gainz", because I've not trained for years and years, I was back to square one. When you start again it really shocks your body and it acts accordingly by quickly building muscle (Providing you're eating enough). I've got maybe another 4 weeks of these "Quick" gains before things really start to slow down. That picture you've seen, I put on roughly 16kg, but it took me 3 years!!