first bike period. thats the biggest issue. on dirt you atleast learn clutching with left and frt braking with right and rear braking with right foot. these are all things that riding on the street you should not have to conciously have to remember. it should be a reaction. you know you got it when your driving down the rode in your car and have to stop fast and your right hand strtches out for a brake
on the street you have tooo many other things to worry about besides which brake do I use, what gear am i in, clutch first then brake, if i go left do i push on the left handle bar or the right? There are many more things that dirt ridding atleast prepares you for but i wont go on.
on the street beginning with a large bike handicaps you to a degree. sure you can do it but...a smaller one allows you to master manueverability without fighting a 500# bike at the same time.
much more could be said but im saving Mr bogus some breath