I personally don't agree that you have to make a cake to feed 100 people. I would call her and tell her that I can only provide a cake to feed 50 or whatever you're comfortable with. This is your gift for the baby shower everyone else gets to decide how much they want to spend and so should you.
I agree with this. If you offered to make the cake, then its your discretion how big it should be. They can supplement the dessert with something else too... wasn't YOUR idea for the cake to be the only dessert!