Steveo1982

Ok so before anyone bites my head off please take in account that im very new to this and im only asking for knowledge purposes. Why is it on a pedal bicycle the big sprocket is upfront where the pedals are and the smaller sprocket is in the rear but on a motorized dirt bike or street bike whatever the small sprocket is upfront with the motor and the larger sprocket is in the rear? Like I said I'm new to this and I'm just wondering the difference in the two