From Ex illis wiki
Bulk (BU) represents the physical size of each warrior in the unit. This affects the likelihood of being hit by ranged attacks as well as the Accuracy penalty someone suffers when trying to shoot over a unit and the maximum number of attacks that can be performed against each miniature in a single round of combat.
Bulk is reduced by 25% when an infantry unit moves and by 50% when a cavalry or flying unit moves. This reduction is not applied when the unit charges or retreat from a zone.