Probably a case of diminishing returns. There is only so many observation and attack units in a given theater that you can deploy, before the enemy starts catching onto the idea of what your strategy is, what numbers you can field, and begin placing assets of their own.

Personally, I think the ratio should be higher for quadcopters and other land-oriented drone units. The ocean is big and usually empty, with the odd concentration of highly valuable targets. If I were to decide how to approach things, I would make the naval drones relatively few, high quality, and stealthy, interdicting key zones of travel and transit. Land territories have to deal with much greater randomness, so you need a greater mix of disposable assets.