The main factors that determine the size and weight of a binocular are

objective lens size and focal length,
prism type and size,
eyepiece configuration,
construction materials used.

Binocular size will vary from pocket-size compacts to large, tripod-mounted astronomical and marine models. Binocular size will generally increase directly with the increase in objective lens and prism size (a 8 x 42 is larger than a 8 x 24) whereas magnification has little effect on the size of most binoculars.

Porro prisms are usually larger, but not necessarily heavier than a roof prism of the same configuration. Construction materials such as polycarbonate/foam and die cast aluminium are used to make a binocular rugged and lightweight. Some models are provided with rubber armour which usually adds about 2 ounces to the weight.

A large part of nature observation is done under twilight conditions and therefore, a standard size binocular is our first recommendation, because they deliver the required exit pupil. Compact binoculars would be the choice where small size and light weight are the most important considerations.