I think the person was laughing at the idea that 'oh no there's a case in this episode, that must mean it's light on Sam and Dean!' I don't know what heavy would mean, then? Sam and Dean sitting around and talking about the good old monster hunting days? (But not talking too much about the monsters, because we don't want to distract from their relationship).
Re: 8.12 spoilers
I think the person was laughing at the idea that 'oh no there's a case in this episode, that must mean it's light on Sam and Dean!' I don't know what heavy would mean, then? Sam and Dean sitting around and talking about the good old monster hunting days? (But not talking too much about the monsters, because we don't want to distract from their relationship).