Safety is an issue but so is your space. Even with offering to move you that still left your mom and whoever replaced you with less space. When I fly with my wife we pick the aisle/window with the hope of getting the row but if it is someone with shoulders or size I normally take the middle and simply set my shoulders. My arms dont leave the armrest and my shoulder goes right to the edge. They either fit, get squished into the window by their own size, or a complaint is raised. Luckily I don't have any misgivings about human contact and when I tell them I work with infectious diseases it goes even better. I am tired of people knowing they need to pay more for the space but simply passing it on to their row-mates. By letting it happen you are simply propagating a bad system. As people continue to get larger, this needs to be addressed. I really think extensions should simply not be allowed in economy these days, space is too limited and if they end up hurt/heart attack/stuck in an emergency, they risk their entire row.