Very sorry to hear
We lost one of our 8 week old Pekins last week, very similar description, minus the larger weight.
He seemed to be a bit off when we got him as a 5 day old chick - leg issues, and always panting (regardless of the temperature).
Over the last month, Chunk seemed to be improving - though still less active than our other two (another Pekin and a KC), still panting often and his feathers always a bit out of sorts.
I had been out in the yard with them for a few hours on the morning it happened (I spend several hours everyday out supervising their foraging). I put the three back in their predator secure pen and went inside for an hour or so. When I went back outside to check in on them before heading off to work, Chunk was laying still on the floor, he had passed.
Our ducks are our pets, and are extremely well cared for, they are fed waterfowl specific feed, get several hours of daily supervised foraging time in our yard and such...we just chalked it up to it being 'his time'.
Our other two are healthy and well, but you can tell they are missing their 3rd... and we were heartbroken - I've become quite attached to the little quackers.
Dan