Yeah, space craft are differant to planetary craft. Some things will work out the same, and some will be "strange". Basicly a large ship can be just as "fast" as a small ship, in fact depending on how it is propelled it can, or should be faster. But a small craft is still going to be able to overcome it's inertia with less energy, and that May equal superior acceleration and manuver. The main reason to use small intercepters might be simple cost, it is a smaller risk to send a small intercepter than a cruiser in a battle space that includes fusion weapons, etc...