When adding an image using an HTML img tag, the file size may vary from the following properties: width and height values/styles applied to the image (a pixel is around 3/4 of a point so they may be recalculated to get a proportional image, other size types such as % are handled) and dpi of the image (https://www.phpdocx.com/documentation/cookbook/standardize-image-size); when width or height (or both) aren't set they are calculated automatically from the image dimensions. Setting the correct previous values will output the exact image sizes you need; we usually recommend setting both width and height values and standardize dpi values to get exact image sizes (https://www.phpdocx.com/documentation/cookbook/standardize-image-size).
In a DOCX document, an image size can change when the content is added into specific contents that have sizes too, such as tables or textboxes.
Please also note that using a Premium license you can also use phpdocx_image available in HTML Extended, that includes more options when adding images from HTML to get exact outputs easier.
If you open a support ticket (https://www.phpdocx.com/support) attaching the image, the dev team can generate a custom script (using HTML) and other custom script (using HTML Extended) to illustrate how to accomplish the task using your image, so you can use it as base for your own scripts.