Video using this picker will be compressed, and Photos you've imported may be at a size optimized for viewing on your device and not at the full resolution from the camera.
To get more direct access to your saved Photo Library, to upload pictures and video at the full resolution available:
If you do want compressed video (since it's much faster to upload) on the Settings tab, in the Miscellaneous section, there is a setting to control the quality of the video imported using this picker.