Why are full-frame SLR cameras so big?

I don't remember because I was born in '81. I mist all these great camera's. But I think I was a sad thing that everything was getting bigger and heavier. I take photography as a hobby and full frame is way to big for me to carry around. Even a enthusiast dSLR is to big and heavy for me to carry all day. Not that I am physically unable, but I just don't want to. That's why I like m4/3 so much. Look at the E-M5. It is the same size of a old OM but digital and it delivers good quality. Even the lenses are nicely small.