They also got big because of economies of scale of operation. Making a reactor smaller doesn't proportionally scale down the staff you need to operate it. This is relevant when as many as a third of reactors in the US are not even making an operating profit.
SMRs will require changes to operational regulation if they are to compete.
Indeed. Many microreactors of the past like the PM3A in Antarctica were just too expensive to operate and maintain so they got replaced with oil. SMRs are great for lower capital cost but they won't just automatically be cheap to operate. The nuclear industry has to focus heavily on getting maintenance and operation costs way down. Some of that is regulatory, but lots of it is inherent.
SMRs will require changes to operational regulation if they are to compete.