Do runts (Dogs) tend to have shorter lifespans in comparison to normal sized dogs?

My friend has a bichon frise runt... It is 11 years old now and until today was surprisingly full of enthusiasm and excitement at all times. My friend recently adopted a kitten and the bichon loves chasing after the kitten constantly. The cat even chases the bichon!
The bichon seems to have some kind of problem with its stomach/back of the body and does not even walk much now (since this morning). It will be going to the vet tomorrow and we'll see what happens.

I've read that the average life span of bichons is between 12 and 15 years. Being that this bichon is much smaller than other bichons (I have a normal sized one... 6 y.o.), will it have a shorter life?

Keep in mind that this dog is usually INCREDIBLY enthusiastic about everything and is constantly running around the house (more so than my younger bichon). You would never think it is 11 years old.

Any responses would be appreciated. Thanks guys.

I've known many runts that live the normal lifespan. Also, isn't it normal for smaller dogs to live longer (Yorkies as opposed to Great Danes)? If you or your friend is really concerned you should ask the vet because they'll know more. Hope this helps!

Ther are runts and runts.

A runt may be small and may have other genetic abnormalities related to insufficient growth while gestating.

It is true that smaller dogs on average live linger than much larger dogs. A minor difference in size probably means a statistically insignificant difference in lifespan, well within the normal range