If you're specifically referring to lying about penis size, it's just an extension of a larger internet mindset when it comes to describing oneself. People on the internet will often lie about everything in their lives, because, perhaps, they are not completely satisfied with what they have. They'll exaggerate their looks, their wealth, their connections, etc, knowing that they'll never actually meet the people they're chatting with online, and it will never be verified. It gives them a place to be the person they wish they could be. Penis size is just one "small" topic of a larger trend.