why small-wheeled bicycles are better

Why smaller bikes are better:
 - Bike has bigger wheel-distance (given a fixed length of the bike) and its ride is therefore more stable.
 - Accelerates faster (because the wheels have less momentum) and therefore also decelerates faster.
 - The rack (and therefore the cargo) sits lower, making the bike and the ride more stable.
 - Small wheels are more robust (lower leverage of destructive forces hitting the spokes).
 - Hub dynamo turns faster, thereby generating power more efficiently. (A lower-weight dynamo can be used.)
 - Bike needs less space.
 - Most small wheelers can be folded, needing even less space.

Why small-wheeled bikes work so great:
 - Gear mechanism entirely makes up for smaller wheels, resulting in exactly the same development as big wheelers (but more acceleration, as stated above).
 - Wide tires make the ride smooth and even offer full suspension for rider and luggage.
 - Modern frames offer the same riding positions as big-wheeled bikes: be it sporty or casual, small or tall rider; everything is available.
 - Most components are standardized and are either the same as for big-wheelers or go by their own standard. So the bikes can be repaired and tuned as easily as big ones.
 - For example, same standard as big bikes:
   + Bottom brackets, pedals, and chains.
   + Gearing-mechanisms (execept that the gears are a bit higher). So if you want to tune your gearing by replacing chainwheels or sprockets just take any made for big-wheelers.
 - Example for a separate small-wheeled standard:
   + Wheels come in 16 or 20 inches. The selection of tires for those is virtually the same as for big-wheelers. (Equally the selection of hubs and rims, if you want to build your own wheel.)
   + Front hubs are usually narrower than on big-wheelers: 74mm instead of 100mm. But there are still hub-dynamos available (e.g. from Dahon and from SON).
 - Even with the wheel-size fixed, there are still plenty of bikes types to choose from: highly portable ones like the Brompton, others which offer a better ride, but don't fold as well, and then those who don't fold at all, and thereby combine advantages of small-wheels with a one-piece frame. Many of the non-folders can also be packed up in very small packages (e.g. Bike Friday, Dahon P.A.Q.)
 - Recumbents and tricycles nowadays use also 20" wheels. (And recumbents are the fastest bikes on the planet.)
 
If you use your bike for daily transport (as opposed to sport or leisure), you'll notice that the small size (and the ability to fold the bike) increase your mobility a lot:
 - Partial folding reduces the parking space needed, e.g. fold the handlebars and pedals down when parking in a narrow in-doors hallway
 - Extra points if you have a Brompton: with one simple gesture the bike sits on its rack, takes very little space, and doesn't need a kick-stand. Getting the Brompton to sit is even faster then pulling a kick-stand down!
 - Keep your bike with you all the time. E.g. get a ride with friends and then still be able to bike home. Or take transit and bike the remainder of the way. Possibilities are endless and there's no need to plan ahead!
 - I usually ride the bike over long distances even if there is a subway I could take. The advantage of being able to take the subway back home, just in case, is the thing which matters. 
 
Too many people are unhappy with their bike because it's not useful enough, or they don't even have a bike, because conventional bikes don't seem useful enough. I will from now on offer free rides on my small-wheeled bicycles to advertise their advantages.