Yes, the energy of the explosion is a function of the total energy between the output of the laser and the shield, specifically the shield energy being pointed at the laser origin. That's why they randomly hid some maximum powered, monodirectional shields facing up when they fled the attack. So that even the relatively weak personal shields would still have a large impact. In general use, the shield energy is being projected in all directions, so the amount facing the laser origin is very small. IIRC, an average powered lasgun hitting a personal shield creates an explosion that can range between roughly an HE hand grenade and an HE naval shell. They only went up like nukes because the Harkonen were using the lasguns at max energy, and the shields were tweaked abnormally. And as another comment says, the actual point of the explosion is at a random point along the beam.