The file size will depend on the content, as stedman1 wrote, but also on the format of the photos.
The amount of detail in the photo is critical for the file size - the less structure is in the photo, the more can it be compressed, without losing sharpness and detail.
As to the format - You can save up to 50% of the storage by letting the camera take the photos in the space saving High Efficiency format instead of JPEG. Look at the Settings > Camera > Formats. See: Using HEIF or HEVC media on Apple devices - Apple Support
What is the source of the videos? Have they been taken on the iPhone or synced to the iPhone with iCloud Photos? I am getting "Try again later" for two possible reasons:
- When Photos cannot download the full resolution videos from iCloud, because of a missing internet connection or lack of local storage on the iPhone, or a problem with the iCloud Storage.
- Or when the video is in a legacy format that cannot be processed on the iPhone or has older adjustments applied, that need reverting to the original, before I can add further adjustments.