Increase in Weight demands an increase in Power
To maintain cruise speed an increase in all up weight demands an increase in power.
So, if you cruise along at 2350 rpm at your usual indicated airspeed for the cruise then add fat and heavy friends plus several pounds of baggage to your max all up weight then your 2350 which you may normally cruise around at will be insufficient to give you the same attitude or anywhere near the same airspeed. Although this seems obvious it is rarely stressed in most training documents.
An increase in weight demands an increase in power.