Was she hatched with any other chicks? What do they look like? Every now and then with crosses I’ll get a smaller bird out of the batch. It’s not that common but it happens. I actually have a 3 way cross rooster from this past year that came a little smaller so it’s no big deal.
I would worry...
You have gamefowl they aren’t bantams. OEGB of any kind are not the same.
Occasionally a bird will come smaller. Just the genetics and the way it goes.
Mixed game hen and she’s most likely small because of genetics and the fact she probably has a lot of Spanish gamefowl in her or being inbred one too many times.