Why do PCBs have big interfaces?
What you're looking at is a PC expansion board using the ISA expansion bus. This uses the then-standard .100 inch contact spacing which was widely available. There are a number of reasons for this. First, the connectors were cheap and fairly reliable. Second, when the PC was introduced, DIP package ICs were the norm, rather than surface mount. This means that the large size of the connectors was not a problem, since any reasonable circuit board was also large by today's standards. Finally, the contact fingers had to be fairly large in order to make the contacts physically strong enough (by having enough glue area) to hold up to repeated insertions. 

EDIT - It's worth keeping in mind that the ISA bus was introduced in the original IBM PC - in 1981. You can read about it here.