2 Light Jackets vs 1 Heavy

Well, you can't rally compare them. One is completely waterproof but less breathable and less insulation. The other is NOT waterproof, more breathable and offers more insulation. So it depends on whether waterPROOF is more important to you than breathability and inuslation. I carry all three… a winshirt, a waterprrof shell and a fleece layer. This allows me greatest versatility in conditions handled comfortably over a broad range.

Well in the UK we have a manufacturer called Buffalo Systems, they make an interesting clothing system. I have 3 tops from them – a Windshirt (Pertex @ 234g), a Techlite Shirt (Pertex/Micropile @ 580g) and a Special 6 shirt with hood (Pertex/Pile @ 790g).
The basic properties of the system are they are warm when wet, very fast drying, bombproof (have had my Special 6 for nearly 10 years) and you wear them next to the skin.
They replace the base layer, insulating layer and waterproof layer (as long as you are active they will keep you warm until they dry out). They are designed for function not fashion and are relatively cheap @ £100 for the Special 6 shirt

what is it that makes jackets light the houdini wet out? Is it the water pressure of heavy rain or do the fabrics simply after enough rain (heavy or light0 absorb some water?

"what is it that makes jackets light the houdini wet out?"

The DWR itself is not really that durable, and not very water repellant either. They can wet-out with even a light drizzle after just a few uses.

As for the Buffalo and Parello type garments, they are great when the temps drop, winds blow and it's really wet and cold, but not much fun in moderate to warm rain, and overkill for when it's cold enough that it's no longer raining IMHO. But they have their advocates.