All testing done in the Arena comparing visuals and damage area at varying levels of Increased Area and More Area modifiers.
Grand Detonation, a Dark Quiver node, states: “Using a Black Arrow causes your next Detonating Arrow to deal damage in a significantly larger area.” Thundersteel Arrow, a Detonating Arrow node, also states “When used directly, Detonating Arrow deals damage in a significantly larger area.” and both nodes have a listed area buff of 100% (Thundersteel specifies “more explosion area”, but since Grand Detonation just says “Area +100%” but uses “Significantly Larger area” where the “increased area” nodes in Detonating Arrow state “in a larger area” I assume it’s also a “more” increase)
So, when using Detonating Arrow with Thundersteel Arrow and hitting the middle target, the AoE would hit 5 targets (middle and 2 on each side), removing Thundersteel Arrow and hitting the middle target only hits 3 targets (middle and 1 on each side). Using a Black Arrow should, logically, allow the Detonating Arrow to hit 5 targets again without Thundersteel Arrow, but it still only hits 3 targets.
Considering it might be +100% increased area, I put 1 point in Concentration for 15% less explosion radius, and at 0/5 Well Crafted it would hit 1 target, at 5/5 Well Crafted (+100% increased area) it would hit 3 targets, so at 0/5 Well Crafted a Black Arrow should allow me to hit 3 targets if it’s +100% increased area, but it still only hits 1.
Thus is my conclusion that Grand Detonation is incorrectly scaling the hit damage area of Detonating Arrow instead of the explosion damage area, or it’s not applying at all.