Edolas Lucy said that what Natsu did was basically blast through 100 Charges of the weapon, while what the shop keeper did was the equivalent to one. Natsu went overboard and cranked the power up and blew the whole thing in one go. As for Wendy, i guess ripping the two halves apart like that let loose all the stored magic, as opposed to how the shopkeeper gently opened it, letting a little bit of magic out gradually. Think of it like a trained professional with a gun, one shot one kill, while an unskilled amateur might just blow a whole carriage of bullets and not hit anything.