Chaos ships have generous fire arcs long range, and can bring more guns to bear in a particular direction than an IN ship and more speed. They can sit abeam at long range and have the firepower advantage.
IN ships have more total firepower than Chaos ships, but to use it they need to be firing at two seperate targets in opposite fire arcs. An IN fleet will attempt to close with the Chaos fleet using their prow torpedoes and nova cannon to put out some return fire. In addition their 6+ prows will make them just as hard to damage with weapons batteries when they are closing as when they are abeam.
An IN fleet will probably have taken more damage by the time it reaches the chaos fleet due to the long range firepower the chaos side can put out. However, at this point the balance usually swings back in favour of the IN Fleet. Each IN ship should now be able to fire at two targets, putting out more firepower than the chaos ships. In addition, the IN ships gain a strong advantage by firing at closing chaos ships that the chaos ships cannot get. If the chaos fleet has not done sufficient damage by this point, the IN will probably prevail.