When the motorbikes got better people started to try and ride them off road, this lead to trials and motorcross. Not much changed for a while people simply just rode road bikes off road.After a while people put better tyres one for better grip, also people started to change the gearing on the bikes, more so for trials so that the bike could go slowly but could also speed up quickly.The problem was that the bikes were still so heavy, the next big thing was to try and make the bikes lighter. Small things such as smaller petrol tank , thinner but stronger frame ( different metal )

Now a days people have made bike very light! road bikes are still heavy but lighter then they were , off road bikes like trial bikes have been made extremely light.
My trials bike is so light that i can almost lift up the whole bike with one hand.