Okay, I've been meaning to point this out for a while… Luffy's clothes do not inflate and deflate.
The only, I repeat, only time Luffy's clothes have expanded to match his size is when he went into Gear 3rd in Episode 288.
However, it is more than possible Oda did this purposely to add to the chibi and/or humor effect.
As for the gun hole… errrmm... Normally, a gunshot will just simply unbutton Luffy's shirt.
Nevertheless, that was a fast bullet, and normally, you'd need the bullet to actually move through the target to leave a hole in the cloth.
For example, take some sort of Airsoft gun or any sort of projectile weapon/plaything/pebble/etc. and place a piece of paper in front of your wall. Shoot the paper, and you will see that the bullet will not move through the paper and leave no more than a possible dent in the wall.
Shoot the paper while there is no obstucting force behind it and the bullet will tear it (depending on how fast it is. An actual gun will for sure).
For that matter, all the times Luffy has been shot, it has always been in areas where his skin is close to the cloth. If his shirt was off and someone shot the shirt, I imaging it would leave a hole.