In the case multiple instances are capable of running a Job, the user has no way to configure which instance Bamboo starts by priority.

In the following example, this would be highly useful:
Instance A is a Large instance
Instance B is a Small instance

Job X can be run only on the large instance.
Job Y can be run on large and small instances.

If Job Y is in the queue alone, Bamboo needs to prefer to start instance B, but if instance A is already running then Y can be run there.

However at the moment, there is no way to control this, while you can use custom Variable requirements you can only restrict the Job to run on a single instance. You can't set it to run on both instance A and B, but prefer instance B.

Having prioritization of instances would help save money as to only start the smallest instance for certain jobs (if needed).