The IW still operate under "Grand Companies" but the actual sizes of these is inconsistent (kind of like Word Bearers Hosts or Alpha Legion Cells). Just think of them like better organized Warbands that still give a damn about the word of their primarch. Although there are actual "Warbands" that have broken from the IW (such as the Steel Brethren) and no longer consider themselves part of the Legion. Grand Companies however, though they dont always get along, still claim to be part of the Iron Warriors as a whole under Perturabo. Whereas Pre-Heresy a Warsmith was a Siege Captain of sorts, now that term is equivalent to any IW Chaos Lord and every Grand Company (no matter how large or small) will have 1 at the head.