Lot's of latitude for something being a pocket gun, but my personal definition precludes ANY pistol heavier than (at the most) 14 ounces, and preferably no more than 12 ounces if I can help it.
Using a weight definition of "pocket gun" is the most practical because anything that light or lighter is going to be, by default, small enough to easily slip into a pocket.
And pistols heavier than 14 ounces, even if small enough, are going to drag you pants down on that side and make you look like you are carrying something "significant" in that pocket.
So using a size definition for a pocket gun is fine if you want to see if a gun with "fit" in a pocket, but it's the weight definition of 14 oz. (max) that is a much more real-world and practical way to choose a pocket gun.
Now, a Compact Glock 19 works great as a "pocket gun" for the boys in the hood, because that actually helps them achieve the look they go for with their pants hanging halfway down their buttcrack.