They don't have to be absolutely huge to give birth. My platies are young and I had no idea they were even pregnant and then I found a few fry. I seriously was suspicious that they or the male were sterile. That said, I agree with adding java moss to your tank for fry to hide in.
I have 150 fry right now Had 2 buy another 30 gallon LOL. My hubby is making me a pond to raise the fry in so they grow faster and only the strong genes survive.