Ah. [He's quiet for a few moments, then turns around to cough. And then says nothing for an additional few seconds. Right as Ciel might be thinking about repeating the question, Sebastian laughs again, softly.]
No, it has nothing to do with... ["size". He shakes his head.]
Yours was the darkest. In that room, no one else... They could not have managed it. ["They couldn't have summoned me" is what he means, as much as the context might make that clear.]
[Sebastian reaches out a hand, vaguely with the intention to brush it across Ciel's cheek, in imitation of the way he brushed his tears away that very first night. Because this is an intimate moment and it warrants it? Depending on how close Ciel is the hand might just end up falling back on the cover.]
You had... [hahh] such a desperate, twisted despair.