Back in the days before pocket 9s and 380s about the only choice one had for a deep concealment gun was either a tiny .22 or .25. I bought a PSP25 which is an top quality licensed copy of the Baby Browning. Its surprisingly accurate and in all these years has never failed to function.
That little gun rode in my pocket many times and I always trusted it. Even today when social circumstances dictate that I shouldn't carry, I'll break out the little PSP and stick it in my pocket. I feel adequately armed when I do so.

Bear in mind that the goal is not necessarily to stop the bad guy. The goal is to stop the attack.
With a tiny .25 or .22 you have to change tactics. Odds are the attack will be at bad breath distance. Forget center mass. Stick the gun right in his face and pull the trigger. Several times. If you get lucky, he may drop like a rock.
If not, take advantage of his disorientation while he tries to figure out this sudden pain and blood and RUN LIKE HELL!
Either way, you survive. That is the goal isn't it?