From my experience, it platies bellies can grow smaller or bigger than expected, and it mainly depends on behaviour, length of pregnancy, and a few physical features to know if your platy is about to drop fry.
Behaviour:
• She will be hiding in various places, for example plants or sheltered ornaments
• She may chase some of the males away from her
• She will stay near the bottom of the tank for the majority of the time
Length
• Usually a female will drop fry after around 28 days
Features
• Near the platy’s anal fin, there may appear to be a white spot, that most likely means she is close to labour (within a few days)
• If you look on her belly, you might see the fry waiting in there. If they are developed and ready to be born, you should expect to see eyes. I know it sounds weird but it’s a pretty important factor in knowing whether she is close or not. If you can just she ‘blobs’ then she is not ready to give birth.
If you are concerned she is ready but she is just not dropping, them there are a few ways to induce labour.
• You can increase the temperature to 26 degrees celsius to encourage her to drop.
• You can move her into a separate, smaller tank, or breeding box, which will stress her out and consequently cause her to give birth.
Hope this help!