It's as big as it needs to be for the compatibilities it has.
For example, NR200 is 185mm wide with 155mm CPU tower support, vs this at 206mm wide and 170mm Tower support. NR200 is 330mm long, supporting an ITX board (170mm) next to an SFX PSU (125mm) vs this case with an ATX PSU (150mm) in the same position. The biggest difference is height, which rises 83mm. This is down to the PSU; if you mount an ATX PSU level with the top of the motherboard, it'll conflict with the GPU. So it has to be mounted higher, resulting in case height rising 83mm rather than ~30-50mm vs NR200.
If they ditched ATX PSU support and reduced CPU cooler clearance to 160mm, in theory they could have got the case down to 21L (196×328×327mm). Though that may come at the expense of 280mm rad and/or 140mm fan support. Personally, that's what I'd have preferred. But there's always going to be someone who is wishing the case was a little different (see NR200 for the best possible example of this... half the case's critics wish it was a little larger, half wish it was smaller).
Wow its almost exactly same size as my Raijintek Thetis, which DOES fit an ATX board. And a rad on top, same as this.
So this case is for the slim group of people that want a custom loop in their ITX?
Dropping the board size down from ATX to ITX seems to have freed up a lot of space for cooling. 2x140mm intakes on the bottom, 280mm rad up top exhausting air, 2x92mm or 1x120mm round back. PSU has a fresh air intake through the side panel. Overall, thermal performance should be markedly better. Personally, I'd take that over 2 extra DIMM slots, a few extra accessible PCI slots, and some extra fan headers.