As an effort to understand the H-mode property of the spherical tokamak (ST), a modeling study is presented about the pedestal stability in the very large elongation (κ) regime and how it then varies when the aspect ratio (A) decreases from a high value of the conventional tokamak. It is first shown that, when κ is very large (>2), the peeling–ballooning mode (PBM) eigenvalue spectrum has a complete shift to the n= 1 limit, where n is the toroidal mode number. This shift makes the mode sensitive to the edge safety-factor q(a), resulting in an oscillating behavior of the threshold pedestal height (Pped) when q(a) increases, as expected for the peeling-type mode. In addition, it allows the PBM to couple with the n= 1 external kink mode (EKM) when the normalized beta (βN) approaches the no or ideal wall limit. When A decreases through the major or minor radius, these mode characteristics are maintained well, while Pped has a different behavior depending on whether q(a) is fixed or varies with A. When q(a) is fixed, Pped has a non-negligible increment with decreasing A, mainly due to the increase of plasma current by enhanced toroidicity effect. Meanwhile, when q(a) varies with A, Pped has an oscillating behavior as expected for the n= 1 peeling-dominant mode. If plasma beta or βN has a large increment through the toroidal field reduction with decreasing A, a sudden drop of Pped is also shown to be possible by the excitation of the high-n ballooning-branch modes around the pedestal or the coupling to the n = 1 EKM near the no or ideal wall limit. Finally, similar to those observed in some previous analysis works, a discrepancy is found between the present modeling results based on the ideal MHD code and experimental measurements in the contemporary ST devices, and a brief discussion is given about its possible origin and relevance in future ST devices.