Resistance exercise involving large muscle groups increases cardiac output through elevated heart rate and stroke volume.

Study: Role of heart rate and stroke volume during muscle metaboreflex-induced cardiac output increase: differences between activation during and after exercise

When you squeeze something hard, your heart beats faster to pump more blood. Even after you stop squeezing but keep your muscle tight, your heart pumps more blood with each beat. This study shows both ways your heart responds during and after hard muscle work, just like in resistance exercise.

Study: Effects of resistance exercise alone or with caffeine on hemodynamics, autonomic modulation and arterial stiffness in resistance-trained women

This study showed that doing heavy squats and bench presses made women’s hearts beat faster and changed their blood pressure — which means their hearts were pumping more blood. That’s exactly what the claim says.

Study: Effects of Acute Upper and Lower Body Resistance Exercise on Cardiovascular Response in Adult Women Through Blood Flow Restriction.

When people do leg exercises like squats, their heart beats faster and pumps more blood than when they do arm exercises — meaning their heart works harder. This supports the idea that big muscle workouts make the heart pump more blood.