You can set a video quality separately for video on WiFi and video on mobile data. You can adjust this setting in Settings → General → Video settings.

By default, the quality is set to "Medium".

When switching to the "High" video quality, note that it consumes significantly more data, it loads the processor of your device and therefore drains more battery. 

If you're on a limited data plan (usually when using mobile data), we recommend setting the video quality to "Low". The "Low" quality consumes 3 MB/min at minimum or more.