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With both my Physique and my Magic Circle receiving a nice boost to their strength thanks to my Subclass, I also noticed my soul growing larger, even if slightly. I think this must be because Class and Subclasses are related with the Soul itself. They're imbued into the Soul, and I've noticed this same growth with all my friends as they Ranked Up their Classes.
The same happens to Spirits when they do, but you can see it even more obviously because they're beings made of their own Soul, so you can see them grow larger and evolve. Unlike people who you can't usually see their souls directly.
And well, as I concentrated in that area of my soul, another message popped up. I was right, it did strengthen my soul a little bit!
Ding!
[Your Soul has grown slightly larger and stronger.]
[Your [Psyche]: [Tier 2: Rank 1: God Devouring Abyssal Soul Psyche] has increased to Tier 2: Rank 2!]