Are you sure they're 5 weeks? They look to be around a week or two old, judging by their wing feathers. The Silkies are always going to be smaller, since they're bantams, but they should be much more feathered in by now if they're that age.
I wouldn't be too concerned if they're active, eating and drinking.
If you are seeing the buff orphintons pecking or bullying the bantams maybe you should separate them.
The Buff Orps may not letting the silkies have access to food.
The silkies may also have a slower rate of growth than Buff Orps naturally.