I was beginning to wonder whether this was an potential exploit. I was thinking it could be a great tool for boosting numbers when being chased down by a much larger enemy - but even then it didn't occur to me that I could just spawn endless troops.
I presume the future solution will be to simply split your existing force. You could choose to have your companion set off with no troops whatsoever, but then it'd be partly your own fault if they get taken by looters.
Still, I'd love to be able to assign specific missions to companions, like those already suggested; not just go off and gather more units though, perhaps, when your army gets too big, take certain units back to a fief and add them to the garrison, or escort prisoners, or scout ahead or even go off and fetch some food; either by asking the local villages nicely, or not...
I set up two of my companions around Omor, my only fief, which was then precariously situated on the frontier. I hoped that they would hover around the area and act as some form of patrol or help me form a rapid-reaction-force to any Sturgian retaliatory attacks, but, bless them, they both sought their own personal glory, and quickly galloped off after the rest of the Battanian armies presently picking a fight with the Aserai, because why not...