Is the male chasing the fatter females around the tank? Not just following, chasing and at times it will look a little like they are going to fight, but they tend to keep their heads out of it. If they are doing this then that is spawning. If they are just swimming and chasing and never coming together its just regular chasing. Even though the tank is not cycled IME fish will attempt to spawn. So this is one possibility.
Do they gasp at the surface at all and do they have red gills? Like maroon red not just a little red. If so they are stressed from the ammonia and/or nitrite currently in the tank. To my knowledge the cycle is not something that would make your fish get fatter though.
You may want to research internal parasites. I believe most would make your fish get too skinny rather than too fat, but I have very little knowledge about them and so am not a good source of information about that.
Do their scales stick out like a pine cone? If so it is probably dropsy, which treatment for is often not successful. Again I do not know the treatments for it, but it gives you an idea of where to look.
Is there any way you can post pictures of the fish? If so that might help folks to help you. I know with danios its hard!
IMO it's unfortunate that you chose to cycle with fish, when you can do this quite effectively without causing a living being any harm. Danios are hardy, that is true, but that is like saying "go ahead and beat up little johnny, he's tough, he can take it." Just because they can live through a cycle, does not mean it is not causing them current pain or future harm. I cycled my first tank with fish, and it was the worst decision I have ever made in this hobby. People told me to take the fish back but I was led to believe (by the store) that the only way to take fish back was if they were dead. They got all their fish back dead. I lost every single "hardy" fish by the end of the cycle, and a few not so hardy ones that the store told me were hardy. I now know that a store will take back fish that are alive, and that even if that store would not there are others that would have. The fish I bought paid for my ignorance with their lives. So I am passing that information on to you in hopes of providing good information. My other tanks have been cycled fishless, and I had no losses there.
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