Andrew is correct.
To expound a bit (spoilers for NPR ship design hidden for those who enjoy the journey of discovery): NPR fleets usually have a set of sensors covering resolutions 1 (50-ton, anti-missile sensor), 5 (250-ton), 20 (1000-ton), and somewhere north of 100 (5000-ton, general-purpose). If you work through the sensor math, this usually means that FACs of ~1,000 tons will be detected at about 1.6x the range as 500-ton or smaller fighters. This means that against missile-based NPR fleets, fighters can get closer and thus need smaller MFCs/sensors, can use faster/stronger missiles with shorter range, and so on, compared to FACs. Of course, you can also abuse this knowledge to find the tonnage "sweet spots" (around 145 tons for fighters and 630 tons for FACs, incidentally)... the point is simply that playing around the limits of enemy sensor range with your parasite tonnage has a potentially significant effect on your ability to close in without detection and launch from an optimal position. This is one of many reasons why playing a multi-player-race game can be quite interesting, if every race has its own sensor doctrine then gathering intelligence about sensor types can be critical to aid in designing the right counter-force.
Against a beam-only enemy fleet, then it doesn't really matter and you can go nuts with tonnage efficiency.