A single Checkered Giant/Flemish kit probably weighs as much as all three of those Mini Rex babies put together.

Mass often is part of the problem, since the litters have to keep themselves warm.
I see people in far colder climates than mine talking about successful winter litters, and I don't know of any reason they would need to lie about it, so I have to believe it is possible. I just know that I can't manage it; I've had does kindle in wooden boxes that were stuffed with hay and the doe pulled plenty of fur, cleaned and nursed the litter, and still, they lost them - even in draft-free bulidings . For me, anything below about 50° F is risky; I started taking them inside when I felt like I simply couldn't bear to look at one more dead litter. Some folks use warming plates or heat lamps; if you go that route, be careful to keep cords where the rabbits can't reach them to minimize the electrocution/fire hazard.