Fast forward to the end of 2021, and she still loves her 128 GB MacBook Air, despite the fact she has hundreds of GB of pix in her Photos library.
I have been looking into a few of my Macs, and it seems that Apple's Optimize Storage function works quite well if you have enough iCloud space. The only problem is that you can't set the space utilized. I'd prefer to have that option to set its space utilization. It seems to like to use roughly 1/2 of your disk space if your data exceeds your disk space.
For example, I have a 12" 2017 MacBook 251 GB and 2014 Mac mini 500 GB, similarly configured, but both with "Optimize Storage" turned on. My Photos library is in the 500 GB range, and Photos access is turned on for both these machines. BTW, the only reason I got 500 GB for the Mac mini was because the 500 GB NVMe drives were only US$6 more than the 250 GB drives, which makes the 250 GB drives almost pointless.
The MacBook has 148 GB free & 103 GB utilized, with 75 GB of that 103 GB used by Photos. (28 GB everything else, plus another 5 GB which seems to be missing out of 256 GB in the way macOS reports it.)
The Mac mini has 261 GB free & 239 GB utilized, with 170 GB of that 239 GB used by Photos. (69 GB everything else.)
Meanwhile, my kid's 2009 MacBook Pro with 120 GB drive has 82.5 GB free & 37.x GB utilized, with 6.5 GB of that used by Photos, which is I believe the entirety of her Photos library. (31 GB everything else.)
P.S. What is the "missing" storage on Macs used for? Is it wear leveling? I don't think it's for the recovery partition because my Mac mini's third party 500 GB NVMe drive is actually reported as 500 GB, and has a recovery partition. My MacBook's soldered SSD is advertised as 256 GB, but only shows 251 GB. Similarly, another machine with 128 GB Apple OEM NVMe drive shows 121 GB, but my machines with third-party 120 GB 2.5" SATA drives show 120 GB.