Harley measures engine size in cubic inches or CI but for my example I'll use CC.
My Road King is 1700cc and weighs close to 900lbs and is a 6 speed. The 6th gear only comes into play when you're hitting speeds at 75mph + which drops the RPM from 3000+ to around 2800 allowing for a more comfortable cruising.
If this size of bike seems ridiculous consider that they are meant for comfort when riding long distance whereas a smaller CC bike would not be nearly as comfortable for those long trips. They would be buzzy at high speeds and you're gonna feel every bump, rock and crack on the road and at the end of the day you will be more fatigued. The suspension on my bike is incredible and it rides like "buttah".
So to a person who rides a bike for practical reasons (maybe their only mode of transportation) if you went on 2500 mile trip on a 125cc, you'd probably be looking for bigger bike when you get back.
Also, my bike has true cruise control which I call a "game-changer". I will always have cruise control on any bike I own in the future.