In managed sql instance, you pick the max memory by the pricing plan. Once you pick a plan, you are charged for the memory whether you use or not, so there is no reason to lower. There is a minimum memory included with each core, but you pay for additional. You can reduce this additional memory if you are using any in your plan.
https://azure.microsoft.com/en-us/pricing/details/azure-sql-managed-instance/single/#pricing