I dont think they're true Bussard intakes. If I remember correctly, one of the major costs in the Dragon was the miniature Bussard intakes. I think the newer ships have a non-100% effective version or similar tech (which accounts for the increased fuel capacity, and the older ships have an extremely primitive, ineffective version (just enough for cruising speed). Truly effecive Bussard intakes (Im not sure if the smaller ones are actually Bussards) are either really big and powerful or just plain expensive.
BTW, they seem rather lacking on some Kilrathi ships...
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You deal death with your roars and your screams, your threats, your taunts, your overblown ego. I hand it out, one with the steel and the silence, the blackness around me, with a thought.