I'd wager the target size of all iOS devices, with the scanning happening by default right after the photo hits the device, is bigger than the other platforms combined.

Google Photos (+ Google Drive I guess ?) is on by default only on Pixel phone I think, and not by default on Samsung phones. For Facebook you have to post the images, for Microsoft I don't think there is any scanning inside Windows itself and by default, I guess it's on OneDrive ? Those companies are big, but the majority of pictures still go through iOS first and foremost.