The smallest pistol I can shoot well (that I have tried) is a Kel-Tec P11, which is a 9mm that weighs 14 oz. empty and holds 10 rounds in a double-stack magazine. I have a pinky extension on the magazine, and feel this is essential to shooting it well.
This gun is easy to carry and conceal in a Fobus paddle holster. I also have a couple of pocket holsters for it. It will easily fit in a jacket pocket, or even in the front pocket of a looser fitting pair of jeans or in the back pocket of a pair of carpenter jeans, like a wallet, using a pocket holster.
These guns can be had for about $300. Some people hate the long trigger pull on this gun, but I think of it as the automatic safety feature. I don't want any manual safeties on any defensive gun. Some people have had trouble with it jamming, but I polished the feed ramp on mine to a mirror finish and it is 100 percent reliable with Remington Golden Sabre 124gr.
Some people will dish Kel-Tecs, but I feel this particular model is a good gun, but could sometimes use a little tuning. The KTOG forum is a great place for support, and Kel-Tec is well known for great warranty service.
For a long time it used to be smallest and lightest 9mm made, but now others have copied it, and perhaps done a slightly better job. I have not shot any of the new crop of very small 9mm's.
I think of this gun as a mini-Glock. It is very similar. Glock does not make a gun this light and small.