Atrophy of muscles happens very quickly...that's why there's a saying "Use it or lose it."
Lowering the weight and adding reps (depending on the range) changes the workout to a muscle stamina rather and muscle hypertrophy workout. Without knowing your body composition (body fat %, bmi, etc.) and the changes, no one can give you a definitive answer to your question.
Three things for sure:
1) if your running and then lifting weights, then your exercising in the wrong order. You need to do strength before cardio.
2) 70% means RM15 - which is VERY light. You aren't going to get your muscle mass back up with that.
3) Pigging out isn't going to decrease your muscle mass - it may add a layer of jelly on top of your Arnolds, but nothing more.