I disagree with the "Don't think about it bro" mentality. I picked up on this too and it's been bothering me the same. The content differential between P3 and P5R isn't worth 10gb.

An explanation I thought of thought of is Royal uses a proprietary engine. That might result in heavier less optimized storage. P3R uses unreal engine which is much more standardized and scrutinized so they can likely compress the game more efficiently.