The efficiency of a DC generator is governed by how its two kinds of loss behave with load: constant losses that stay the same and variable losses that change with current.

The constant (fixed) losses are the iron losses in the core and the mechanical losses of friction and windage; these are present even at no load and do not depend on load current.

The variable losses are mainly the copper (I²R) losses in the windings, which rise with the square of the load current, so they grow rapidly as the machine is loaded.

At light load the copper loss is tiny, but the fixed losses still consume power against a small output, so the efficiency is low; as load increases the output rises faster than total loss and efficiency climbs.

Maximum efficiency is reached at the special load where the variable copper loss becomes equal to the constant loss. Beyond that point the copper loss keeps rising steeply while fixed loss is unchanged, so efficiency falls again.

This is why efficiency neither rises at a constant rate, nor stays flat, nor falls steadily from the start; it follows a rise-peak-fall curve.

Hence, the answer is Efficiency will initially increase then decrease after a certain point..