Why do plants' leaves become enlarged in low light areas?
I have no idea of the biological mechanisms behind what I'm about to propose. Consider this a hypothesis.

If the purpose of the leave is to collect sunlight for photosynthesis, then the behavior you mention is predictable. I order of trying to maintain a healthy flux of light in dim conditions, the plant adapts by increasing the area of its leaves. Once back in a well-lit environment, the need for larger leaves fades and new leaves are thus smaller, but still catch as much light. Of course, there must be limits to the extent of leaf area variation that one plant can handle.

As for the stem structure, I can only guess it's because plants spend more energy on leaf building than stem maintenance when under such stresses.