For one thing, as I've adumbrated, the ager publicus tended to wind up not in the government's hands, but in the hands of the conquering generals and friends of the conquering generals.
Tiberius Gracchus tried to bolster the citizen military by taking ager publicus from the wealthy and giving it to the landless poor. This was of course interpreted as trying to buildup a clientele.
Overall Roman armies proved so successful that they defeated more enemies and conquered more territory that they wound up with more ager publicas or public land than they really knew what to do with.