Even iPad users should pay attention to this issue, despite the 9.7' screen size for iPad and 7.9' for iPad mini. You'll feel comfortable placing the device a bit further away while still being able to read the content clearly on the screen.
To enlarge subtitles and captions on iPhone, iPad, follow these steps:
Step 1: On the main screen menu, go to Settings > General Settings
Step 3: Choose Subtitles & Captions
Step 4: Select the Style for subtitles
Step 5: Here, you have various options. Usually, simply choose Large Text
If you want to create your own subtitle style, you can choose Create New Style. Apple provides numerous customization options: Font, size, color...
https://Mytour.vn/enlarge-subtitles-iphone-ipad-172n.aspx
With simple settings, you can easily increase the font size for subtitles when watching movies, videos on iPhone, iPad. For iPhone users, this is a genuinely useful feature as the iPhone screen is only 3.5' to 4'