User blog comment:Marco the Marine Admiral/Theory: Evilness Can Lead to Power-up for Dragon Slayers/@comment-24066070-20140628165509

The thing about the four DS you mentioned is that all of them have a Lacrima imbedded in their body. So they likely took some power form the Lacrima to fuel their own Dragon Force.

It was stated that DS couldn't consume the element they generate themselves (Natsu can't eat the fire he creates) or even if they would do, they won't get a power boost. So likely this "law" applies to DF generated from the Lacrima. Their body may consider it to be consuming their self-generated element, thus resulting in a weak DF.

Natsu and Wendy on the other hand consumed powerful elemental magic (Flame of Rebuke and Ethernano), thus they got a powerful DF.

The power of DF may be determined by the "quality" of the element they consume, if the body considers the magic from the Lacrima as self-generated elemental magic, it would result in a less powerful DF than when they consume a very powerful, external magic.