Semis get the most story share. That's not always the same as being the most powerful, but between the skills (including combat) and the magic, they have something to do more often than anyone else.
An Arms character can be extremely good at something, but they lack versatility. In combat, they will typically be very good at killing one foe at a time. Whereas a mid to high level caster may have attack options, but also battlefield control, defense options, buff spells for support, summoning, etc, and they can likely attack multiple foes at once. Even in combat they are more versatile. And outside of combat, their spells replace a lot of skills, so even more so. Storywise, information spells are often more effective in advancing party goals than skills are. To address that, you have to make skills more important.