@BlackstripeI mean, bear two things in mind. For one thing, Mira is so sheltered that she probably has no idea that Homunculi are even a thing, and for another, her Magic Circuits are INSANELY good by modern standards because she's a Witch, even if she doesn't have many of them. So, given what she knows about the degradation of Magic over time, there's really no reason for her to assume that there's another Master who can rival, match, or exceed her level of output.
I'm not really sure how the two of them compare power-wise, though. I think Mira has more raw Prana at her disposal, but Estelle has her beat when it comes to efficiency due to being literally a living Magic Circuit, so she could definitely output more energy at once, making it possible for her to overwhelm Mira in the short term.
EDIT: Also, I hope I managed to do justice to the Nasuverse tradition of weird fight scene descriptions. I felt like having the traditional "Master sees Servants fighting for the first time" moment was non-negotiable. XD