 Solution

The helium yield is primarily neutron-limited, so small changes in cosmological baryon density have only a weak effect compared with their effect on residual deuterium. Increasing the baryon-to-photon ratio makes deuterium destruction less effective relative to its formation and releases the deuterium bottleneck at a slightly higher temperature. Nuclear reactions also proceed faster. Fewer neutrons decay before capture, so a small increase in baryon density slightly increases helium-4; a decrease slightly lowers it.