Kinetic energy from a projectile is equal to the mass of the projectile times the square of the velocity, but air resistance saps velocity off quite rapidly and independently of the mass. So a heavy projectile will deliver more kinetic energy to a target than a lighter one with the same initial velocity.

A lighter projectile can be driven faster by the same charge of propellant, but air resistance increases with the cube of the velocity so delivering more energy through increased velocity is a losing game. You can decrease air resistance by decreasing the cross section of the projectile, but that requires dense materials like lead.

tldr: the difference is absolutely not minuscule. There’s a reason why bullets are made of dense materials.