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 for this increase in strength and not size is because of the way your muscle fibres work. In order for you to grow muscle you need to activate more muscle fibres you can achieve this for example with explosive weight lifting movement like the snatch which leads to greater muscular gains increase the muscle mass 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 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, the more ropes that are awake and active the bigger the whole muscle will be.