You can now right-size your message broker by modifying the instance type on demand. Instance type changes can be applied immediately or during the next maintenance window. Vertically scaling your message broker provides flexibility to adjust for seasonal changes and increase capacity as your application grows. 

from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/09/amazon-mq-introduces-vertical-scaling-for-message-brokers/