'what do you do b4 you start working the bag?
do you have a decent warm up?
decent stretch, holding it 15 sec and do everything twice?
losen up your joints and dont forget to rotate your wrists witch is important.

after this you should start of easy on the bag, building up the power.
since your a big guy, your hands and wrist get alot of impact witch they are not used to,
so build up slowly with the power punches.

you might as well consider using sparring gloves on the bag, the heavier ones (16+ oz).
the offer more protection to your hands than regular bag gloves.

and after you finish punching the bag.
do some easy and very lose shadowboxing to cool down.
then rotate and losen up your joints again.
and stretch all the muscles twice 15 sec.

this should keep those hands from hurting.
hope this helps