If you want to see how big a photo you took with your iPhone really is, you’ve come to the right place.
iOS has many tricks on its sleeves, like gestures or swipes that help you get things done. One of those tricks is showing the size of a photo.
When you have a photo opened on your screen, just swipe up, to open a section full of data regarding that exact photo.
Among those, are the resolution and the actual size of the file. You can see, for example, that a photo taken with an iPhone 12 can take around 5MB.
Eventually, you can opt for a conversion with a photo editor like Pixelmator, that and reduse the size without a big quality compromise.
With the same swipe up you can also see where a photo was taken.