I still won’t believe you if you said there was a valid reason for putting in a smaller disk in a machine - and even less if you say it happened! 
What you’re talking about is repartitioning systems that are generally and often multi partition systems. DOS / Windows was multipartition since the earliest days…
But I’ve NEVER seen single partition systems changed. Linux often does a separate partition like /home, but not SME, nor NethServer.
Also, I’ve never seen anyone want to shrink a partition on a PFsense or OPNsense. A larger disk, eg to accomodate Proxy, sure! But not resizing a partition!
Why would you want to, the pre-sizing of both OPNsense and PFsense make sense as they are!
But bad planning exists, that’s also true. Even, and especially in large Enterprises! Waste goes so easily unnoticed in large organisations…
And: We’re talking about “servers” here, not workstations or gaming machines in a private household!
My 2 cents
Andy