The dimensions of the Mac mini have remained unchanged since 2010, despite multiple internal revisions. Apple has removed the 2.5-inch drive and the DVD drive, while a teardown video has even slimmed down the size of the motherboard for the new M1 model, raising the question of why it has kept its mini-PC the same size.
https://www.notebookcheck.net/New-Mac-mini-teardown-reveals-that-Apple-could-have-made-its-mini-PC-a-lot-smaller.505090.0.html