Helena is more strong-willed than Hermia, and is also taller.
The main reason that Helena seems stronger than Hermia is that her problem is more urgent. She needs to act to prevent Hermia from marrying her Demetrius.
Helena and Hermia were friends when they were younger, and both are from highborn families. Helena is very strong-willed and Hermia is meeker.
Hermia seems to be reluctant to engage in difficult situations. When her father brings her before Theseus and insists that she marry Demetrius, she just goes. Once there, it seems to take great courage for her to speak up.
I do entreat your Grace to pardon me.(60)
I know not by what power I am made bold,
Nor how it may concern my modesty,
In such a presence here to plead my thoughts (Act 1, Scene 1)
Hermia does not tell her father or Theseus that she does not want to marry Demetrius. She stays quiet. She is not a complainer.
By contrast, Helena seems a bit unstable and definitely is strong-willed. She jumps to conclusions, and she is not afraid to speak her mind at all. All Hermia has to do is say hello to her and she seems to jump down her throat.
Call you me fair? That fair again unsay.
Demetrius loves your fair. O happy fair!(185)
Your eyes are lode-stars and your tongue's sweet air
More tuneable than lark to shepherd's ear,
When wheat is green, when hawthorn buds appear. (Act 1, Scene 1)
Since Helena has been jilted by Demetrius, it is understandable that she is upset. While Hermia seems to go with the flow and only acts when Lysander suggests that they leave, Helena takes action. She tells Demetrius that Hermia and Lysander are going to run away, hoping that she can get Demetrius to see her point of view and realize that he loves her, not Hermia.
You could also refer to their physical strengths, which could be metaphorically related to their emotional strengths. Helena refers to Hermia as “little” and Hermia calls Helena “maypole,” so clearly there is a height difference. Yet Hermia is strong and willing to fight, with enough provocation.