  The expansion of Web of Science Core Collection (WoSCC) over the recent years
has partially accounted for the "norm" of growth of research output in many
bibliometric analysis studies. However, the expansion patterns of different
citation indexes may be different, which may benefit some disciplines but
hinder others. Utilizing Science Citation Index Expanded (SCIE), Social
Sciences Citation Index (SSCI), and Arts & Humanities Citation Index (A&HCI),
this study attempts to elaborate on WoSCC's coverage expansion patterns among
these three databases from 2001 to 2020. Results show that different from
SCIE/SSCI, both the annual publication volumes in the A&HCI database and all
A&HCI journals have remained relatively stagnant in all document types
considered scenario or have gained relatively slight increases in only citable
items considered scenario. Although the number of A&HCI journals also has
increased remarkably, the average journal publication volume of A&HCI journals
has decreased gradually if all document types are considered or kept relatively
stagnant when citable items only are considered. Besides, the A&HCI database
has ceased the systematic index of individually selected items from SCIE/SSCI
journals since 2018. The study finally discusses the possible causes and
consequences of the unbalanced expansion of WoSCC's different citation indexes.
