I would be speculating if I gave a reason, but I think we have seen correlations between family socio-economic status and obesity. We know that generally family socio-economic status is lower in Atlantic Canada than in other parts of the country, so you might be able to make that association as one particular connection.
The other thing is that generally the demographics of Atlantic Canada are that it's more of an aging population. This would be more for the adult rates of obesity.
A lot of younger people leave Atlantic Canada. But I think childhood rates of obesity might relate to socio-economic status, and the availability and affordability of healthier foods.