Difference in width of permanent and primary incisors is

Incisal liability  When the central incisors erupt, these teeth use up essentially all of the excess space in the normal primary dentition. With the eruption of the lateral incisors, space becomes tight in both arches. The maxillary arch, on the average, has just enough space to accommodate the permanent lateral incisors when they erupt. In the mandibular arch, however, when the lateral incisors erupt, there is on the average 1.6 mm less space available for the four mandibular incisors than would be required to perfectly align them. This difference between the amount of space needed for the incisors and the amount available for them is called the incisor liability. Textbook of Orthodontics, Gurkeerat Singh, 2nd Ed, pg-566