Why Safari on iOS12/13 automatically reduced my selected photo in HTML5?

I use Safari on iOS 12/13 device, when I uploading a huge size photo(size > 7MB) with input label in HTML5.

Code Block 
// HTML5 input label 
<input  style="display: none"  id="input_image" type="file" accept="image/*"/>

The photo I get in JavaScript, only 1 / 10 the original size(e.g. Origin size is 8.1MB, JavaScript get only 725K)

Code Block 
input_image_dom.onchange = function(e) {
		// calculate size label
			var show_size = document.getElementById("show_size");
			var file = e.target.files[0];
			var size = file.size;
			if (size > 1000 * 1000) {
				show_size.textContent = (size/1000/1000).toFixed(2) + 'mb';
			} else if (size > 1000) {
				show_size.textContent = (size/1000).toFixed(2) + 'kb';
			} else {
				show_size.textContent = size + 'byte';
			}
			
			uploadImageFile(file)
		}

We do a lot of test with on various devices，all have same issue.