Not wrong, but “a few millimeters” means a lot more for a 14500 light than for an 18650/21700 light. While many may think the ~8mm difference in length between the D4V2 and D4K isn’t much, it’s enough for some folks (myself included) to sacrifice the runtime.

More importantly, it’s harder to accommodate a wider range of lengths without getting some rattle on shorter cells. Not an insurmountable problem, but definitely a consideration. The range required to take protected/USB-charging cells is a bit wider; >10% of the cell length, and a much larger percentage of the travel length of the spring. Dual-spring helps by splitting that across two springs, but has limits.