In that case I would strongly recommend changing how you squat. You can get away with almost anything with 45 kg on the bar. That'll change very quickly when you're squatting even 70 kg.
Focus on keeping as rigid a trunk as possible, from the back of head down. These are the steps that work for me, compiled from tips given me by some heavy squatters and good coaches. I find it helps to do them in order, but you'll figure out your own way. It starts before you unrack.
- set the bar on your back (low or high, your call)
- pull the bar down into your back, try to bend it over your back. Squeeze the bar.
- take some air into your belly and squeeze down on it.
- arch the bar out of the rack
- walk the bar out. Take as small a step as possible.
- let the bar settle
- drive your head hard into the bar
- take as much air into your belly as possible and squeeze hard
- pull the bar harder down into your back
- pull your elbows under the bar
- start your descent - depending which works for you either drop straight down pushing your knees out OR pull your butt back pushing your knees out and pushing the floor apart with your feet (that works better for wider stance IMO which is what I do)
- stay tight during your descent, which will generally mean slowing it down a bit
- when you hit depth reverse direction fast leading with your chest and keeping your elbows pulled under the bar while driving your head into the bar and pushing your knees out.