There are a handful of system-level settings in iOS that can affect the size of text in Sixcycle.
Settings > Display & Brightness > Text Size
Make sure the the slider is in the middle of the range of options
Settings > Display and Brightness > Display Zoom
Make sure this is set to "Standard"
Settings > General > Accessibility
Make sure that "Larger Accessibility Settings" is off, and that the slider is in the middle of the range of options
In all likelihood, it's one (or all!) of these settings. Get them set according to the recommendations above, then reboot your iPhone to correct the sizing of text in Sixcycle.