Wild Sourdough Starter Never gets more than 1/3 bigger - Am I doing something wrong?

I made a wild sourdough starter back in July of last year. Did some baking with it, but it has never doubled in size the way that I see described in articles online.

I initially started it with all-purpose flour and aired-out tap water, but I’ve moved it to Bread Flour and Spring water. At best, I’ll see it increase in volume by roughly 1/3.

I keep the Sourdough Starter in a pair of Ball Wide-Mouth pint jars, and feed the starter twice per day. The method I’ve been following for the past month has been:

• Stir the sourdough starter thoroughly.
• Pour 2 oz (56g) by weight from the old jar to the new jar.
• Add 2 oz of spring water by weight to the new jar. Stir thoroughly.
• Add 2 oz of Gold Medal bread flour by weight. Stir thoroughly.
• Mark time and initial level on new jar.
• Cover with two coffee filters, and screw the lid rim on to hold them there, and allow for outgassing.
• Lay the lid insert on top to reduce drying.
• Wash the old jar to get it ready for use as the new jar in the next feeding

Temperatures in the apartment are typically about 75-80°F by day, 80-85 by night. Starter typically peaks in 8-12 hours,

These images were taken a few weeks ago, after about 8 hours after feeding, when the starter was peaking. The black line is the original level after mixing, and the blue line is at about the peak. As I said, I’ve never seen my starter increase in volume by more than a third.

Why isn’t my starter doubling or tripling? Am I doing something wrong? Could I remedy this?