The High Yield YouTuber speculated that the M3 Pro was basically simplified so that the silicon designs for the base and Max could be more easily re-used for the Pro and Ultra [1].
Not an expert at all. But I think gist of the video is that GPU size relative to other component has increased. Plus they rearranged some of the "furniture" to accommodate that increase.
Have you watched the full video? He talks about the re-use from 8:49 onwards in a section called "Why is the M3 Pro a downgrade?"
Actually I did but I was mostly focused on the GPU. That was the most fascinating aspect for novice like me.