Why do we use a DB9 connector for serial com?
why does rs232 use such a big connector?


Assuming you mean the standard 25-pin connector, then there is a simple answer: The RS-232 specification states the defined function for every one of the 25 pins bar three. There are lots of features of RS-232 which are little used these days!

This diagram is a brief overview of the pinout of the 25-pin connector:



(Figure above courtesy of Dallas Semiconductor Application Note 83, Fundamentals of RS–232 Serial Communications)


  Why do we use a DB9 connector for serial com?


It's a similar explanation for the 9-pin connector made popular by its use on the original IBM PC/AT (the earlier PC and PC/XT used the full 25-pin RS-232 connector). All 9 pins have a defined function, whether you use that function or not.

(Strictly, it's a DE-9 connector, not a DB-9. The first letter, D, refers to the D-shaped connector's metal shell. The 2nd letter is the size of the connector shell, and this smaller connector is size "E", whereas the larger 25-pin connector is size "B"; other sizes exist too. See the Wikipedia article on the D-subminiature connectors for more details.)



(Figure above also courtesy of Dallas Semiconductor Application Note 83, Fundamentals of RS–232 Serial Communications)


  We only use 4 wires (pwr, common, and two data)


In fact there is no power pin in the RS-232 specification. Of course people can adapt an RS-232 connector to include a power connection, but then it doesn't fully meet the RS-232 specification.