As Brian says, larger sprocket at front = less load on the chain, and the 18T as fitted to K1 - K6 bikes is the largest which will fit at the front. The original fitment to the very early bikes was actually a 16T front which Honda theirselves superceded to a 17T as a result of chain / crankcase braekages, before going to 18 / 48 from the K1 onwards.
I actually run my '69 with an 18 front / 45 rear. Keeps the revs down a touch when cruising, but still pulls well enough ( and if I need more acceleration, thats what the gear lever is for).
Now if traffic light grand prix' are your thing , try a 16 front / 48 rear!
Chris R.