2 things I can guess (I'm having the same problem as you right now):
1. The light level is not high enough anymore to support the finer leaflets
2. The plant ages to a point at which it begins to produce the broader leaves instead
I honestly don't know which or if both play a role, but like you I am seeing much larger (less attractive IMO) leaves and have honestly thought about completely removing the wisteria from my tank (due to its massive growth).
I've had the wisteria now for over a month and a half, and its probably doubled in size well over 4 times (most likely much more but I've lost count). At least once a week I'm clipping and throwing out huge handfuls in my 20gallon tank.
I think the clippings you cut off and more importantly where or how you replant probably also plays a large role in how the plant grows.
About another month or so and I'm going to seriously trim back the wisteria to only 4 or 5 branches using only new leaves and see if it maintains the fine structure or if it goes back to the less attractive leaves.
What I've done now with the wiseteria which I find to be very useful (both in looks and hiding equipment) is to cut the plants so that they are flat in a plane (i.e. like a piece of paper), and then using them side by side along the back of the tank as a background. These remain completely flat and hide the heater and filter intake for over a week before I start to see growth coming towards the lights (forward that is), at which point I clip them off and try to maintain the flat backdrop.
I'll see if I can find a pic that shows this and post one.
justin
EDIT:
This isn't a great pic since its after the wisteria started to grow towards the middle of the tank (and probably right after I cut it 50% down since it normally is right near the surface hiding the entire back of the tank) but you can get the idea: