This paper briefly reviews the engineering development of Modelica-based hydraulic system modeling. On this basis, it reconsiders the issue of reasonable decomposition in component-based hydraulic modeling and discusses the methodological relevance of Xu's method for the development of Modelica hydraulic model libraries. Finegrained component decomposition facilitates rapid assembly and reuse, especially for users without extensive hydraulic modeling expertise. However, excessive subdivision may also introduce duplicated pressure states, hidden volumes, artificial fast time constants, increased numerical stiffness, and ambiguous physical semantics. Through order-of-magnitude examples involving pilotoperated relief valves, pressure compensators in multiway valves, short manifold passages, hydraulic cylinder chambers, and accumulator groups, the paper illustrates that the appropriate component boundary depends on the modeling objective, the dominant physical mechanism, the available parameters, and the quantities to be validated.