When do you need to start worrying? DS is fifteen months and an absolute tubster. Probably due to shoot up shortly I'd say, it tends to go in cycles- omg he's tubby-ooh he's grown-omg he's getting tubby again ad infinitem...he's got a VERY healthy appetite. 90% of what he eats is homemade, healthy, balanced with the odd treat thrown in, he just likes a lot of it. I swear if you could translate his toddler distance to adult miles he'd be running a marathon every day so I try not to restrict him too much.
I think that's a little young to worry. They're often chubby at that age and thin out now they're running about more. Keep portion sizes appropriate so he has time to digest a bit and see if he's actually still hungry and asks for more, or just eating it because it's in front of him.
But worth weighing and measuring from time to time to keep an eye on the various percentiles so you know where he is and particularly any trends. Depends what you mean by tubby too. If he's so tubby it's inhibiting his movement it might be a problem, but it doesn't sound like that's the case?
three of mine were so fat they were spherical, one looked like I'd inflated him with a bicycle pump. Now he's a lean and healthy 6ft 1 14 year old. I was more worried about the skinny one.
I think toddlers are supposed to be chubbier, but are you feeding him all the time? I know obese children where I live and I live in a small town in the developing world, the only food they eat is mostly home grown/locally produced food. Homemade and balanced is not the issue, it's the quantity that is causing the weight gain, not the type of food.