Because electronics companies and people have been pushing the miniaturization of electronics since they knew how. As a result, we get greater efficiency and power, with a smaller form factor. The trade off is that the R & D could take longer and cost more.

With most tech and electronics though, bigger isn’t always better, because with decreased form factors you can make room for other things, that will increase efficiency and power. You see it now with smartphones which are literally mini pocket computers, amongst other things.