Can't figure out planet smashing, am I the only one?

I've built several systems now for the express purpose of smashing multiple asteroids into a planet. Unfortunately, none of the systems I build end up having asteroids that are movable. From the live stream on the 11th it was mentioned that you can only move objects with a radius of less than 300 so in the system I built today I tried implementing this (three bodies with radii between 150 and 250) and still no option to move the 'roids. There must be something I'm missing. Is there someone in the community who can show me the way and grant me the gift of exploding planets??

The only alternative I have to give you is try to reduce the mass. For me every planet with 0-300 requires 3 engines and 25 one you actually hit 300.

Here's a tutorial on how to smash planets: http://pamatches.com/2013/how-to-destroy-planets-with-the-halley/