Lifting weights slowly is a great way for your muscles to gain strength, but is not a great way for the muscles to increasing with size.
The reason why slower exercise is for increase in strength and not size is mainly because of the way our muscle fibres work. In order for you to grow muscle you need to activate more muscle fibres, in order to activate more fibres you need to do more explosive exercises like compound weight lifting movements (clean & snatch). These exercises use more muscle fibres and explosive weights combine with movements will give you greater muscular gains increase the muscle mass, and faster than other training methods.
Why the muscles work this way ?
For example, imagine that our muscles fibres are little ropes, in order to pull more weight you need more ropes and the more little ropes you have the bigger and stronger the whole rope is. A muscle is made from thousands of little ropes, some which are awake and some which are asleep. You need to wake your ropes for muscle increase the more ropes that you awaken the bigger the muscle will be.