Did you read the part which says "micro", these are about 2-3x the size of Pi with those cheap plastic cases. I had mine beside the router before I moved it inside TV console.
Pi will be single purpose device, the "micro" can be full blown server and comes with ALL parts required, no need to hunt for good PSU, case, cooling, storage. Some of them come with 1 Sata and 2x M.2 NVMe, so you can literally pass those NVMe to TrueNAS in proxmox and run full blown NAS with encryption/encoding/decoding in VM.
May be you mean 2-3x in every linear dimension but that still seem to be smaller than those in my mind. (While Mac mini is not known to be the smallest, it definitely exceeds 2-3x in some linear dimension already.) What kind you have in mind?
Pi will be single purpose device, the "micro" can be full blown server and comes with ALL parts required, no need to hunt for good PSU, case, cooling, storage. Some of them come with 1 Sata and 2x M.2 NVMe, so you can literally pass those NVMe to TrueNAS in proxmox and run full blown NAS with encryption/encoding/decoding in VM.