Hermia completely misses the point the play makes that love is blind. Therefore, when she realizes Lysander has abandoned her and fallen in love with Helena, she grasps for a rational explanation (don't we all?). She decides Lysander must have fallen in love with Helena because she is taller. She shows the sudden loss of self-esteem Lysander's scorn has left her with by calling herself "dwarfish" and "low."
This is an example of dramatic irony, in which the audience knows what characters in a play do not. None of the four lovers—Hermia, Lysander, Helena, and Demetrius—realize that the male's new love-besotted pursuit of Helena comes from a fairy love potion.
By this time, we as an audience understand that love is changeable, like the moon—and definitely does not make sense.