Traditionally, binoculars providing both the degree of magnification and image clarity required for detailed observation, have been relatively large and weighty objects that you hung round your neck on a strap or lanyard.
As optical manufacturing techniques have improved, and optical coatings have improved the transmittance of light through the optics of binoculars, so the reduction in size of binoculars has become more feasible. Compact binoculars now offer improved portability with little or no loss of performance.
This is largely due to the adoption of the roof-prism or Dach design as the means for transferring light through the binoculars.
The roof prism design splits the light into two paths and has a more complicated light path than the Porro prism design. This traditionally meant that the precision with which the roof prism was manufactured and assembled was a significant factor in the cost of such binoculars. This meant that whilst they were lighter and more compact, they were considerably more expensive than the Porro prism designs.
Previously most binoculars were manufactured using the Porro prism design. This design is bulkier and weightier than the roof prism design, but not so technically demanding in the quality and precision required in the optical manufacturing process. This gave a price and performance advantage that the roof prism could not, until recently, compete with.
Advances in coating technology, especially since 2005, and the improved accuracy of optical manufacturing processes, have meant that, especially for the higher quality compact designs, the Porro prism design, no longer holds the advantage.
Binoculars using the roof prism design are more compact and lighter than binoculars of similar specification using the Porro prism design. They have, typically, a straighter barrel for each side of the binocular and weigh considerably less than the Porro prism counterpart. This permits a much more compact binocular, delivering as good if not better performance, than it's bulkier and weightier competitor.
Certainly, for the majority of general purpose uses, the more compact binoculars permitted by the roof prism design, are much more comfortable to carry about, typically fitting into a shirt pocket, and are much less tiring to hold to the eyes, with little or no loss of performance.>