I think, and I hope somebody corrects me if I'm wrong, but it relates to ultimate size of the fish PS at 7-9" vs BG at 10-12" and recruitment with PS laying up to 14,000 eggs per season and BG laying up to 60,000 eggs per season. Those numbers vary a little bit on the web from what I could see. Larger fish spawn greater amounts.
In the case of SMB with a smaller gape more of the PS will survive and ultimately recruit.
In the case of LMB with a larger gape less of the PS will survive and ultimately recruit.
Couple that with fewer eggs per season PS compared to BG and you could end up with a shortage in the forage base.
Just me guessing though....hopefully one of the pros will chime in.
Pumpkinseed Eggs per SeasonBluegill Eggs per season-------------------------------------------------------
Exactly