When I was a kid my mom's cat got out and got pregnant, she was very very small. She ended up having 5, but the 5th one got stuck and wasn't breathing. My dad did kitty-cpr and got him breathing so they all ended up ok, but we had to help them learn to walk straight. They were so cramped inside her their legs weren't quite right. She did fine during the birthing, but me and my dad stayed with her the entire time to make sure she didn't have any problems.
My mom-in-law's cat got pregnant a few years ago, also very petite. She ended up with 6 completely healthy babies and the only issue we had was having to break the sack on the runt. Again, we stayed with her the entire time to watch for problems. Mom-in-law didn't get her fixed and her and her sister went and got pregnant again, at the same time, and they ended up with 11 babies between them. The second cat was only about the size of a 6month old or so, and we had to combine their kittens because she wasn't making enough milk for them. If all goes well during the birth you should still keep an eye on that since they don't have any reserves to hold them if you don't catch it.
Hope it goes as easy for you, we had so much fun even if they were unexpected