Diesel was a bit over 2 lbs at 6 weeks when he came home. He's 7 months now and nearing 50lbs.
I guess I should add to this. He was the runt of the litter, (the whole litter seemed small though) although he was the smallest he was a spunky little guy. After his first vet visit, they discovered he had coccodia, which the vet said stunted his growth a bit. I was very worried about his weight and size, but now I see how big he's gotten, and I had nothing to worry about. He's bigger than some other boxers his same age. So I'm sure you have nothing to worry about. Just make sure they're cleared by the vet.
He also wasn't too fond of hard kibble when he came home, so he would only eat the wet food. So I gave him as much as he wanted every half hour when we went through the kitchen to take him out. He would only eat a tablespoon at a time of food. But by his 8 week vet visit, he had gained 2 lbs. and it went steadily up after his coccodia cleared up.
So all I'm trying to say, is get them checked out by the vet, then just enjoy the puppy stage. I'm sure there's no reason to worry.