> However, if you compile on an AWS EC2 instance (as you may want to do if there is a cloud component to your fishtank), then you get a 16-byte structure because int there is 8 bytes

I'm not familiar with any OS that EC2 would offer that uses an 8-byte int. A better example might be that long is the size of a pointer on Unix and microcontroller systems, but always 4 bytes on Windows.