My FBCM hen started molting a month or so ago (is it too early?)
and as soon as she started moltingshe stopped laying eggs i hope to get at least one dark egg when she starts to lay again
I also bought another pullet from a different breeder that should start laying soon
The new pullet is significantly bigger than the hen i have
Is that normal ? It is also a purebred FBCM
Exactly when a hen starts to molt will depend on many things, so it is not necessarily too early for them to be molting now. Most breeds and lines will molt in late summer to early fall, as the day length decreases (assuming that you are in the U.S.), so she is probably right on schedule. Many of my birds are molting now also. It is normal for them to stop laying when they molt. For birds that are not winter layers, they will often molt in the fall as the day length (photoperiod) decreases, then restart again in late winter, as the photoperiod increases. You can somewhat manipulate that by putting lights on automatic timers in their coop to start their mornings before sunrise (don't extend the photoperiod in the evening -- you don't want the lights to suddenly go off when the birds aren't on their roost, leaving them on the floor in the dark). Adding lights will get them to start production earlier than normal in the winter, but they do need at least a 2 week break after the molt and feather regrowth is finished.
Different lines of "purebreds" will appear different, and some will be much larger or smaller than others. Birds that are bred to the SOP will all be similar size, as the Standard calls for each breed to have a specific size and conformation. However, hatcheries don't breed to the SOP -- they just breed for production because that's what is profitable, so most hatchery birds are very undersized. New breeders that start with hatchery stock won't have their flock up to Standard for many generations (typically more than 5 years, often much more), and breeders that don't work towards the SOP will never achieve that goal, but often sell their birds as "purebreds." Being a "purebred" does not mean that the bird represents the breed well -- it just means that someone claims that no other breeds are bred into that individual bird. To get a bird that represents the breed well, you need to know what the breed should look like and do in every aspect, and then seek out a breeder who has birds that accomplish that goal as closely as possible. But no bird will perfectly match the Standard in every way in both appearance and utility, even the ones that are champions, so be sure that you have reasonable expectations.