I think you will send yourself crazy if you go down that road, and deffinatly should not be blaming yourself!
How fantastic that you managed to warm up the second born! 4 beautiful kittens! You must be soooo proud of mum!

I am no vet, but i really don't think the size of your cat would have been a problem, my cat fell pregnant at 3 1/2 months and gave birth to 5 kittens. (i lost one as he had a cleft pallet, but this is nothing to do with her size).

Chin up hun, just take joy in your georgus little ones!