If you bring up a couple of default image VMs in the default VPC in EC2, they will generally have their interface MTU set to 9001. If you sent TCP traffic between those two instances using their private IP addresses, it is easy to verify that full 9K jumbo frames are used (e.g., by examining interface stats and calculating the packet size as "TX bytes / TX cnt").
If you send data between the two instances using the public IP, or just out to the internet, it is similarly easy to verify that an MTU of 1500 bytes is being used.
So everything "just works" - but how?
How is the MTU of 1500 selected for packets that will traverse the internet gateway? There's nothing set up in the routes that would enable this, and all incoming ICMP is blocked by default (and in these tests) by the security group so PMTU should not be working.
