All pups & dogs should be wormed on a regular basis, especially in the first few weeks of life.
When two of my cats were kittens (back when they were fosters
), I had the pair of them after the others from the litter were adopted.. well the one cat was VERY VERY skinny and underweight an to be honest I thought he was going to die for the longest time.. his brother was great, plump and healthy. Then, when they both hit about 9 or 10 months (after we realized they were never getting adopted LOL), they did a total 180; the skinny guy is now super fat and the chubby one is quite lean. Both are in "healthy" weight ranges by my vet though. The one that was fat before is lean and muscular. The one that was skinny is a BIG guy now, like thick and chunky.
Weird how things work.