It's far easier to think that planet probes are easier to make via passive material collection than it is for bullets, railgun ammo etc. are. Maybe the planet probes are very simple things to make - simpler than an explosive bullet. Maybe the material required for a planet probe can simply be passively collected from the interstellar medium, the sparse dust and gas in the not-quite-total-vacuum of space.
I'm more inclined and more willing to take that explanation - seems a handful of others are unwilling to do so.
If each probe was the size of a grain of rice an imperial cutter couldn't contain an unlimited amount.