Heavy weights for mass and light weight for cuts is one of the biggest myth in bodybuilding. 
Let me explain to you, There are two types of muscle fibers in our body.
1. Slow twitch muscle fiber(TYPE 1)
2. Fast twitch muscle fibers(TYPE 2)

The fast twitch fiber responds for heavy load and explosive movements. This is the muscle fiber which has the capacity of growing fast. So when you are bulking up, try to do more compound movements which can help you gain some solid muscle. The slow twitch fibers respond well to longer duration of stress(i.e.,high reps) these fibers do not grow as fast as their counterparts but if you miss out these in your workouts, your body wont look balanced. So for optimal muscle gain and balanced looking physique do a combination of heavy compounds and isolation movements.And remember this, “Diet and rest play equally important role for gaining muscle”.

Doing 1000 reps without weights wont bring in cuts. So you have to use optimal weights(resistance) if you want to work your muscles optimally. For displaying cuts, you need to get rid of the fats under the skin.So to get cuts – diet, cardio and workout is the key and light weight has noting to do with it.

And finally, dont get carried away by the words of some gym doctors who have not done anything other than advising fellow gymmers. Get the help of a qualified health professional.