The size of a Derringer that is in a cartridge of relative effectiveness is fairly close to the size of some smaller revolvers and semi autos that would likely be considerably more useful in a fight.
Think about this, If you are down to your backup, you really are in a bad way and absolutely need a handgun that is going to work 100% of the time, firing a cartridge that is going to put an end to the trouble you are in, versus a feel good talisman, which is what most derringers and mini revolvers really are.
Do you really want to be in a gunfight with a NAA single action mini revolver, with its 1" barrel and non existent sights, against an opponent who is armed appropriately? Why choose to deliberately handicap yourself? Gunfights happen fast. They are ugly, and you need every advantage you can get. Choosing a derringer is not unlike choosing to load blanks in your gun while the other guy holds all the advantage. Screw that.
Yep, guns can be uncomfortable to carry. Beats being dead or crippled for life. I have a former co-worker who is now in a wheelchair because he let his guard down. Others who are dead.
A 5 shot J Frame, Glock 42/43 is about as small as I would go as a BUG, and not as a primary. The question I always ask people is "what is the smallest gun you are willing to get into a gunfight with 1 or 2 people with?". Don't go smaller than that.
That may not be the answer you wanted to hear, but it is what it is..