DS is disgruntled because we don't want to host Christmas day. They hosted about 7 years ago, their choice. We haven't had them at ours.
This would be for their family of five, our family of four, and 4 - 5 other family members (maybe 6 at a push).
Our house isn't really big enough to comfortably accommodate 15 people or seat them for dinner. We also have two dogs (medium sized) and their three children are scared of dogs. We would put them (dogs) elsewhere as they never see these people so would become a bit excitable, but if we said "please don't go in X room because of the dogs" it wouldn't be respected and that isn't fair to the dogs, to the kids, or quite frankly, to us. They would be continually unsettled if people walked in and out and then we'd get nagged at if the children were scared (the dogs are friendly fwiw).
AIBU to not want to host? DH feels awkward as we have been asked if we will be hosting this year and I have said I don't want to. I'm happy to have everyone around for 2 - 3 hours, then go for a meal and go our separate ways, but I'm not cooking for 15 people, trying to squeeze 15 people in to my house, and shutting my dogs away in one room all day because of the hysteria the children descend in to when they see a dog.