28c is usually the magic size as it give you enough purchase to grip while skidding, and usually have enough rubber to last a decent number of time.
When a tyres get smaller, it limited the amount of rubber it can hold, and the internal would be very small if the company decided that the rubber should be as thick as it's 28c version.
Touring tyres in 23c can >>>>>>>>>>>>>>
28c is usually the magic size as it give you enough purchase to grip while skidding, and usually have enough rubber to last a decent number of time.
When a tyres get smaller, it limited the amount of rubber it can hold, and the internal would be very small if the company decided that the rubber should be as thick as it's 28c version.