Is lifting heavy necessary for getting big?
Here’s another question for the ages. Simply put no, you do not need to train “heavy” to put on muscle. That said you do need to use enough weight to stimulate the muscle to grow. So using 2lb db’s for 100 reps is not going to butter the biscuit. However if you are not training g to be a power lifter reps of 1-3 are not necessary either.
People need to leave their ego at the door when they walk into the gym to train. The most important piece to hypertrophy training is time under tension meaning how long can I lift things up and put them down without breathing? There should be variations week to week of heavy( 6-10 reps), moderate (15-20) and light (20-60) including drop sets and forced reps. That’s a basic over view for you.
So their needs to be enough weight to stimulate growth, enough time under tension that you can control with “tempo” (another discussion) to cause the muscle to need to adapt.
This is just a very brief overview on how to approach your training. Training is very individualized and like diet you need to play around a bit.