The thrusters are working as intended. The difference in optimised masses and the mass of the ship itself when fitting various thrusters can result in non intuitive results. At the end of the day if you're more under the optimal mass then the C will provide more of a benefit than the D one. Likewise if you're over the optimal mass the C won't punish you as much as the D one will. Also the C has a higher integrity rating so that it can take more punishment before failing.
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