I have had a good broody and a bad broody. My bad broody smothered and killed all her chicks and refused to stop setting.... It happens. Not all broody's were meant to be moms. Funny thing, my mom said that the chicken who smashed all her chicks would make a bad mom because she sat too tight when ever my mom tried to gather eggs she had stolen.
The chicks should be fine as long as she doesn't step on them. Another thing that can kill the chicks is suffocation. When breed is she? If you are really concerned, you should take the chicks away untill they are fluffy.
Several possibilities. Some hens are simply very careless mothers with heavy feet and their idea of warming the chicks is to literally sit down fully as if they were still on eggs. A good mother hen actually hovers and is quick to ever so lightly raise her body when she feels movement. It doesn't have much to do with weight or size of the hen- there are silkies that are extremely poor mothers and I have a big and very fat Marans hen that was the most excellent mother.. just looking at her it's hard to imagine SHE could be a good extremely delicate and attentive mother but I entrust valuable eggs and babies to her.
Other possibilities are persons who are excited about the new babies and just have to peek under while the eggs are in the process of hatching.. understandable but some hens respond to intrusion by getting "tense" and dig their legs in against the eggs/her body and also press down in a defense of sorts. That can push the eggs against each other.. any with weak spots which includes pip holes can get crushed during this. It's very advisable to leave first time mothers completely alone during the hatching process.. there is the occasional first timer who associates hatching with intrusion and never really gets her natural instincts allowed to cycle towards accepting the chicks happen uninterrupted.
Other hens are what others said- they are more setters than mothers.. they have trouble leaving the nest and would much rather sit on a new round of eggs than leave the nest with the chicks. Those usually make not very good mothers even if forcibly removed to a mother pen with her babies. They still will brood and care for the chicks but...