Quite big tiny particles in this application: Xenon is a fairly hefty atomic number of 54 - exactly double iron.
And you need quite a bit of it: even fairly small spacecraft like probes can have nearly a tonne of the stuff. Which, considering there's only 30-40ish tonnes extracted per year at a cost of about 1.5ish dollars per gram is quite a bit!
Ions are small enough that you can bring enough for a whole trip pretty easily. Yes they're still consumable, but you need a tiny fraction of the reaction mass you need with a conventional rocket.
And you need quite a bit of it: even fairly small spacecraft like probes can have nearly a tonne of the stuff. Which, considering there's only 30-40ish tonnes extracted per year at a cost of about 1.5ish dollars per gram is quite a bit!