I know this is meant as a slight against the gunt rather than a condemnation of Xander, but I have to disagree. All babies (all healthy ones anyway) will go through phases of chub. Growth comes in waves and babies store up their energy for those spurts so one day you'll have a fat happy baby and the next you'll have a taller, somhow heavier, skinnier happy baby. That's been my experience so far anyhow.
Chub is also more pronounced in formula fed as opposed to breast fed babies. Xander being Faith's first kid and Faith being the way she is I'd put money on him being exclusively formula fed. From what I saw of his limbs in the picture he's in good health. By far not the fattest baby I've seen. He'll still end up fat because his main diet when it counts will consist of fast food and simple sugars, but he's not technically doomed to inherit the gunt quite yet.