if you feel around their chests, it should feel like a half circle from the bone running down their front. If the half circle feels more like a divet with pudge coming out of either side, your duck might be a little overweight.
As babies, it's okay for them to be a little pudgy. They will need it to finish growing.
I have a runner who is pretty chunky. she's an infamous food stealer, stealing all the treats of the other ducks. Her but is pretty huge... She was 3lbs the last time we got weighed at the vet... We were betting on how much she'd weight now, we're guessing around 4.2-4.3 lbs. lol. She also gets a huge "eggnant" belly too, making her look extra chunky. If you think your duck is a little too pudgy, the best thing to do is reduce how often you feed them, or feed them smaller amounts more often throughout the day. You can also let them free range, and give them healthy treats like greens, rather than peas or corn.