Smaller payloads are often custom made, it’s not a stretch to make a bigger one also especially considering that it’s a thing that already happened previously. I’ve seen video of Ukrainians melting TNT out of mortar shells (it can be done with hot water) to put it in lighter, custom made drone payloads, surely there’s a supply of TM-62s or loose explosives to use in this way

otoh sounds like a good thing, because it suggests that 155mm projectile supply is adequate