I think what Callistra is trying to say is that any food that expands a lot in water has a lot of wheat gluten or other grain in it. Protein doesn't suck up a lot of water, grain does.
To understand why this isn't great, we know that bettas' digestive systems are not made to digest lots of grain/carbohydrates. When food containing lots of grain is injested, it still does suck a lot of water out of the fish (it will still expand a bit before it's digested). Since bettas aren't designed to do this, it'll cause constipation and bloating.
Foods that are protein based will not expand as much no matter where it is, in the water or in a stomach. The betta's digestive system is designed to handle this, so it's not a problem. Bettas don't have enough fluid in thier system to handle a lot of grain and grain based food...the water is sucked out of them faster than they can replenish it. Funny how this can happen with fish, but it does.
Does that make any sense? We need to look for food that is the closest we can possibly get to what bettas would eat in nature. They eat protein, with a bit of green matter(whatever is in the stomachs of the insects they eat). There's no carbs in there, so carbs is something you'd want to avoid. My hikari betta gold has wheat as the first ingredient. I tossed it, since I don't want to be constantly worry about constipation. But I'd only guess that you can continue to use that food and constipation would only be an occasional thing. Some fish tolerate it better than others.