Tortoise growth

[Koopa1]

Hello, I am getting quite worried about one of my tortoises called Seb as he/she doesn't seem to be growing a lot compared to my other tortoise koopa. Seb was only born in 2015 however I don't know the day or month. Koopa was born in 2014, they were both the same size when I first got them, koopa has grown and is looking very healthy however Seb doesn't seem to have grown at all and also has a quite a flat shell compared to other tortoises I've seen. I've attached some photos, hopefully someone can help!