Help- Chicks too small/not growing

[Cmbeck22]

I was recently given five 6-week old chicks. They are Silkie bantams, which I have raised before. They are way too small but they are feathered and have so much energy.
They were raised on 20% protein Chick Starter, which is what I've feed all of my chicks, however they were not under any heat lamps and have been outside since they were one-week old. I'm worried that the lack of heat made it difficult for them to grow, and if they don't grow, there's no way they survive the fast-approaching Canadian winter.
Is there anything that I can do to have them grow so late? I'm really not set up for heat lamps anymore (that's why I take them at 6 weeks, not hatching) but I could set something up if I had to. Will that do any good? Or is there something else I could try?

 

[SilverBirds]

Silkies and other bantams in general are slow growers. What makes you say they are too small?

 

[Cmbeck22]

I compared them to pictures of my previous flock at 6 weeks, and they are much smaller. Each one can fit in the palm of my hand. I've attached a picture of one, though I know it's hard to compare size through pictures.

 

[Tretinker]

I'm afraid I know nothing about this breed but can you insulate your coop in anyway? Making sure it is out of the wind would help too, if it's out in the open hay or straw bales could be put up around a couple of sides.
When does it start to get really cold for you guys?

 

[ChickNanny13]

They could just be smaller than the stock you have.

The feathering on the one in the picture, looks like "regular" feathers & not all "fluff". Are the "new" ones all feathered with regular feathers? Those should be able to "insulate better" than those with the feathering the Silkies have. Still an insulated coop would benefit all your Silkies, due to the type of "feathers" they have, they don't insulate like regular feathered ones. They don't do good in the rain either, the "fluff" don't keep them dry like regular feathering. Could we get a picture of your Silkies?

https://silkie.org/types-and-sizes-of-silkie-chickens.html