User blog comment:IamJakuhoRaikoben/Chapter 433 & 434: Ikusatsunagi & Demolition Fist, Review./@comment-1776824-20150512000613/@comment-1776824-20150512220559

Yes, their increased strength depends on Lucy. It was said specifically in that chapter that her Spirits will gain power as she gains it. Nothing was said about them having a negative power correlation, only a positive one. You don't summon a powerful spirit and then have it automatically become weak because Lucy is. What happens is that the Spirits gain power by drawing it out of Lucy, which also affects the amount of time they can spend in the human world. I will use Aquarius as an example. Aquarius is (and was) by far Lucy's strongest spirit, was she ever weak? Not in the slightest. However when Lucy gained more Magic Power, Aquarius was affected because she could pull more out of Lucy. A mage's magical strength does not hinder the power of their summoned being. If they can summon it, then it comes out with its base strength at minimum. However, if they have excess that, then the spirit can borrow that power and stay in the human world longer. It isn't a hard concept.