Doesn't matter if he or she was the runt, genetics come into play of who is behind them in the pedigree and their size. Jrts can have very large skulls and c-sections are not uncommon.
My last c-section bill was over $750.00 and of course there were added bills later for other problems. With lost wages, that litter cost me thousands, actually it was close to five thousand and that included the stud fee for 3 pups.
And there is also the risk of Eclampsia which is common for small breeds when in whelp and especially during lactation. If they feed a cheap food that risk is much higher. If they don't know the signs and don't seek immediate Vet care, the bitch is guaranteed to die..........orphaned pups now to raise.
If the pair only bred once, risk of a singleton pup...........c-section.