I have 2 males and a female. They were all purchased at the same time and the female has grown much larger (at least 1" more) than the males, she has a greater girth as well. The females generally do get larger from what I've read. Yours is probably carrying some eggs - if you want to try and induce spawning, do a 50% water change with 3 to 4 degrees cooler water than your tank temperature (assuming your other fish are hardy and can handle the temperature change). I've yet to save any eggs - the danios and platies get to them first.