Page "Magnetic mirror" Paragraph 4

This result may be surprising because one might intuitively expect that heavier and faster particles, or those with less electric charge, would be harder to reflect, or that a smaller magnetic field would reflect particles less efficiently.

However, the gyroradius ρ = mv / qB in those circumstances is also larger, so that the radial component of the magnetic field seen by the particle is also larger.

It is true that the minimum volume and magnetic energy is larger for the case of fast particles and weak fields, but the mirror ratio required remains the same.