Answer: Apparently the variety of ivy (Hedera helix) you are growing is one which produces large dark leaves at maturity; some varieties will maintain a smaller leave size and/or a lighter green over time than others. Unfortunately there is no easy way to "prune" ivy. If you want to force new growth, you will need to cut back into the vines in order to force fresh new growing tips to develop. To do this, start at an existing growing tip and work your way back into the vinery, tracing along a main stem, and cut it. (New tips will start behind the cut.) Untangle and remove the trimmings as best you can and repeat. As you can guess, this is tedious at best.
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