Actually, a folding bike can be fitted to give you PRECISELY the same saddle height and cockpit length as a full-sized bike. With a little practice, most Dahon models can be folded up in 30 seconds, about the time it takes to correctly lock up a full-size bike.
A folder functions just as well as a full-size bike. I talked with two woman who used folders for a 300 mile tour, with heavily loaded saddlebags.
A "really light" bike (weighing under 20 pounds) costs over $1,000 new...and parked on a New York City public street is likely to be stolen within a week or two.
A college teacher I talked with was tired of worrying about having his full-size bike stolen, he keeps his folding bike under his desk.