Your PSU has an efficiency curve. Chances are that you use more power with your 550 watt PSU compared to a 750 watt one, since it has a lower efficiency while supplying the needed power.

I don't know how efficiency curves for modern PSU look like, maybe it isn't too relevant. I think most PC are well served with their optimal efficacy at around 300W.

Here is a curve, but I don't know if that is representative of reality:

https://linustechtips.com/uploads/monthly_2017_09/efficiency...

This PSU would be most efficient when your system draws 250 - 300W of power.