Luman had told the twins that the further they went into the Swamp of no Return, the stronger the Demon Beasts would become. However, Luman also mentioned that he didn't know if he had truly got further into the Swamp or not during the time he was lost. It's just that there was a time when Luman found many stronger Demon Beasts as he kept moving everywhere.
It wasn't wrong to say that the further you enter a Demon Beasts' Territory, the stronger they were. After all, that's how most of their territories worked. If you are stronger, then you have the power to fight for better resources, which are usually located deeper inside those territories. It was basically logic.
Even humans followed the same path. The stronger and wealthier you were, the closer you lived to the center of a big city. Even a small village with no more than a few hundred people would usually have its chief hut at the center of the tribe.
