Your ducks are lovely. I cannot tell from the photo if you have males or females. Nor can I know if they are jumbo.
My son has pekins he raised from a few days old, two females and a male. One female is crested and smaller than the other two.
Currently have 4 pekin drakes [one is actually my son's drake who joined us 6 weeks ago.] One of my pekin drakes is crested and smaller.
We also have muscovy. The non-crested pekins [male and female] are about the size of the smaller muscovy drake. None approaches the size of my larger muscovy who is both physically big and very heavy. [He's a great lump of soft, cuddly drake!]
Both the crested pekins [male and female] are a little larger than my son's muscovy female, but she is small by muscovy standards.
All pekin ducklings grow and put on weight quickly, far quicker than our muscovy ducklings did. But the pekin ducklings stop growing sooner -- at 2-3 months, whereas our muscovy carried on increasing in weight to 3-4 months.
To keep pekins healthy, just be sure to give them extra niacin -- even if they are on duck food, duck food is optimum nutrition for the average duck and pekins have greater than average needs. I add 1 tblspn brewer's yeast sprinked on top of each cupful of duck food I put in their bowl.
Whether you have males or females: most male pekins have a drake curled tail feather by 2-3 months old. One of mine molted his and it hasn't as yet grown back; the other lost all his tail feathers fighting with my muscovy. He is now getting new tail feathers growing back, but doesn't yet have a drake feather!! My two new rescues both have drake feathers. If you can take photographs from the side of your pekins' rear ends, we will be able to see if htey have drake feathers. But even if they don't at 8 weeks old, they may soon get them