Small or large a bike is a bike, The trouble starts when you get sloppy with your riding. This can be down to any number of things. But Generally the physical weight of a small bike makes it easier to get away with sloppy riding . This Is not to say bad technique is acceptable on small bikes its just that if things get out of shape its possible to avoid the off by simple bossing the bike and staying on it. Obviously the bigger/ heavier the bike the less you tend to get away with. Experience as in most things is all important in motorcycling, learning from an early age progressively, is a good thing and the lessons learned will never leave you. But not all have the luxury of learning from childhood, and its not always an option or a necessary pre requisite to be an accomplished rider at the end of the day. The basic control of any bike is the same generally, and i think fine if you have a 125 or a 250 as a beginner you will learn the basics of say stopping and setting off safely in any direction or incline with less drama on a small bike. But you still need to get a feel for the potential pitfalls and learn from your experience. this will sharpen up your riding skills and make the transition to a bigger heavier bike less problematic. This basic control experience i talk about here is at the slower speeds here many of the heavy bike complications i talk of here are encountered. Drawing to a halt at slow smooth and higher speeds, is where the bigger bikes can be a bit of a handful if you don't approach the manoeuvre properly. Stay smooth is the two words which never go out of fashion in motorcycling, smooth is fast and its precise which in turn is safe. If your learning on a heavy bike, Be precise with your technique, because with a heavy bike, get it wrong past its point of no return if you like and believe me your going down. Every type of riding from MX to touring or fast road or race, all need good basic control, the type of control learned at slower speeds like say in motorcycle trials. trials in my opinion is a good basic foundation for motorcycling whatever type you care to participate in.