A canonical statement I have frequently read is that "large diameter axons conduct action potentials at faster velocities than small diameter axons". After recently learning the effect of increased capacitance on the cell membrane (namely, that more charge is required for depolarization to occur...Are large cell bodies of neurons harder to depolarize than small cell bodies of neurons?), I find this canon to be rather confusing.
In the above picture, both neurons have the same myelin spacing. So, my question is why should the action potential in Axon B traverse the distance X1 to X2 more quickly than in Axon A. Shouldn't the increased capacitance of Axon B (due to its increased diameter) increase the time it takes for the action potential to travel between the two points?