” don’t think the sun can expand debris it turned to rubble in the first place?”
No, the point of fact is that your point relies on energy imparted into the debris to do that spreading that is not there.
In other words, magic.
Okay, so then the Roche limit effect somehow shattered the ISON nulceus - shattered it - but those shattered pieces all stick together and keep moving along the same vector without spreading apart, because... the force that shattered it in the first place, only shattered it, but did not impart enough force to move the shattered pieces away from each other.
LOL, now that's what I call "magic"!