The title is a question my 14 year old daughter asked me yesterday after we'd been out star gazing, and as I wasn't 100% sure of the reasons ("something something gravitational collapse... pressure... mass...") I thought I'd Google it and put it here for anyone who's ever wondered the same.
https://www.forbes.com/sites/starts...k-ethan-why-do-stars-come-in-different-sizes/
https://www.forbes.com/sites/starts...k-ethan-why-do-stars-come-in-different-sizes/