Imperial military doctrine evolves quite naturally when you consider what warfare in Star Wars is actually like.

In star wars ships travel through hyperspace. This allows them to quickly, though not instantaneously, traverse the galaxy, generally without impediment. This means that an attacker can suddenly show up with a massive fleet nearly anywhere, and the defender needs to hold out for some time for reinforcements to arrive. Whoever controls orbit around a planet can potentially bombard the planet to subjugate the population, or land troops to occupy the planet, or at least interdict trade, which many worlds are highly reliant upon.

At the same time, the galaxy is massive. Millions of inhabited worlds, many thousands with significant industrial and political power. In the clone wars there is a roughly even split - both the CIS and the Republic need to defend a huge number of systems. While each can field large fleets, those fleets need to be mostly dispersed so that they can resist attacks. Every victory adds a new system to defend, further limiting their capability to launch more attacks. One would expect eventual stalemate, even if the two sides weren't being run by the same guy to produce exactly that result.

For the Empire, the situation is different. The empire controls basically every system, whereas the Rebellion doesn't have to defend anyone. It is free to use all of its forces to attack anywhere, and it does not need to subjugate the planets it liberates. The Empire, therefore, must maintain a force everywhere which is strong enough to take on the entire rebellion (or at least the entirety of any one rebel faction). But this creates two problems for the Empire.

First, any increase in the Rebels' strength dramatically increases the costs for the Empire to field its military. One capital ship might double the strength of a rebel cell, which means the Empire needs to essentially double its whole navy. And second, lots of ships means lots of captains which is the real threat to the Empire. The empire is an extremely centralized government, representing a compromise for the galaxy between their freedom and security. Some portion of the people innevitably are going to be unhappy with that compromise, and the more ships you have, the more likely one of those captains will be among the discontents willing to defect (or sufficiently incompetent that discontents under their command will mutiny). On its own, it doesn't seem like that big a deal, what's it matter if one imperial capital ship in tens of thousands defects? But recall, a rebel cell adding one capital ship doubles the cost of the navy. A defecting imperial ship could be that capital ship.

Thus for the empire to survive, it needs to defend a huge number of planets with as few ships as possible. This logically leads to a focus on large capital ships which can feasibly stand up to a small fleet single-handedly, with fighters that can't act independently and support ships with minimal armament, presenting little threat if they were to defect.

Of course the Empire would be best off if they didn't need to defend their worlds. If they could sufficiently intimidate member worlds, who obviously can't get up and run the way a hidden rebel cell can, then they can tell them "tough titties, endure the rebel attack until we arrive" and avoid parking a Star Destroyer over a world. This would allow for a much smaller navy in terms of both ships and manpower, meaning those ships could be made more capable and all be placed under the command of loyal and competent officers.

The natural progression of this are the super star destroyers and ultimately the Death Star, which could concievably keep the whole galaxy in line. These superweapons serve an additional purpose though. Because they can potentially invalidate the Rebel's strategy and ensure permanent imperial hegemony, the Rebels have to attack these targets. Thus instead of millions of potential targets, the empire now knows to focus their efforts on a handful, and when the rebels come out of the woodwork to make these strikes, the empire can make them especially costly. The Yavin cell risked everything to take out the first Death Star, and came within seconds of total destruction. Likewise Endor was a trap that would have been the end of the Rebellion were it not for those meddling kids and their stupid teddy bears.

It's worth noting that the Empire's playbook worked. They consistently won victories; it was only when their power structure imploded and they descended into infighting that the New Republic was able to make serious gains, and they ultimately had to compromise with the imperial remnant rather than subjugate them by force of arms.