1. They don't put jump drives on all fighters because of size. A jump drive is at least as big as a normal engine.
2. The afterburners thing - the Morningstar had a jump drive. In fact, every ship afterwards that had a jump drive had afterburners. Which just proves number 1. Obviously, there were problems fitting a jump drive and AB's together on one ship.
3. I don't care what your stats say. The Rapier in the movie is huge compared to the Ferret. Even if it's the same lenght, remember that the Ferret is exceedingly narrow. Sometimes I wonder where the pilot puts his legs
. Just compare pictures of the two.
4. The average corvette is 4-5 times the length of a normal fighter. Also, the only Confed corvette that we know anything about - the Venture class, is 80 metres long (compare to Ferret's 10m), and nonetheless only has two engines. That, according to the diagram (hehe... I got photocopies of the WC 1 cap ships from the original WC 1 manual; eat your heart out, KSaga), would give each of those engines a diameter of... 6.7 metres. The engine is thus nearly as wide as the Ferret is long
To make things more interesting, one should note that the 360m long Exeter class Destroyer, only carries four engines, each more than 25 metres in diameter. This indicates that bigger ships use bigger engines, rather than more engines
A Kamekh class corvette in WC 2, btw, has engines nearly 15 metres in diameter.
You will also notice that WC 2 cap ships indeed have lots of smaller engines - but alas, while they seem small at first glance, they too, are at least 25 metres in diameter
. The smallest engines on any cap ship in WC 2 is the Free Trader (freighters generally appear to have much smaller engines), which has 3 metres or so of diameter. Unfortunately, that's still too much for the Ferret.