Suppose you're using one of the the Keck telescopes to look at a distant object. This collects all the light falling on its mirror, which has a radius of 10m. All this light is then focussed into the image plane.
Now suppose a galaxy the size of the Milky Way is lensing the distant object. This collects all the light falling on the galaxy, which has a radius of about 50,000 light years and focusses it.
Which is going to be brighter?
I'm exaggerating for effect because the difference in brightness is far, far smaller than I've suggested above. Gravitational lenses do not have a simple focal plane like the lens in a telescope, and only a tiny fraction of the light they lens reaches our telescope on Earth. The increases in brightness due to gravitational lensing are generally pretty modest. Nevertheless it should be obvious that a gravitational lens can collect more light simply because it's so big.