Thank you for that feedback! It's very useful to understand what the experience is for people across a wide range of perception and devices. I will look into what I can do to tweak the contrast here. Look for changes in the next update and let me know if they've helped!
Regarding size, note that the app has been designed to take into account your text size settings from the Settings app - so the option exists for you to increase the size there and have the changes reflected in all apps that use caption-sized text fonts.