With ridiculously small cases like:
Mini-STX cases:
Silverstone VT01B (1.53L)
Silverstone VT02B (1.88L)
ASRock DeskMini (1.92L)
Thin Mini-ITX cases:
Silverstone PT13B (1.4L)
Realan E-W44 (1.98L)
Luna Design's DNK-H (2.12L)
Mini-ITX cases:
Logic Supply MC500 (2.31L)
Mini-Box M350 (2.5L)
MITXPC MX500 (2.84L)
Micro-ATX cases:
Jonsbro C2 (12.1L)
InWin BK623 (12.5L)
ATX cases:
Streacom FC5 Alpha (8.48L)
Thermaltake Core G3 (23.58L)
And then more gaming focused models we are all familiar with:
Micro-STX:
ASRock DeskMini GTX/RX (2.7L)
Digital Storm Project Spark (4.7L)
Mini-ITX:
NFC S4M (5L)
Dan A4-SFX (7.25L)
NCase M1 (12.6L)
Micro-ATX:
Chimera Cerberus (18L)
Chimera MachOne (18.5L)
ATX:
Chimera Cerberus X (19.5L)
There are MANY other cases in the 1.5-25L range that I'm familiar with and taking into account, but I thought this cherry picked list nicely highlighted my argument without just being exhaustive.
Mini-STX/Thin Mini-ITX: 2.5L
Micro-STX: 5L
Mini-ITX: 15L
Micro-ATX: 20L
ATX: 25L
