icloud photo file size

How can I download a JPG photo from my icloud.com account to my PC (Windows 10) and have it be the same file size? I tried to download a 1.7 MB photo using the download option "Unmodified Original" and I got a 1.1 MB HEIC file. HEIC--What is that!!? Then I tried the modified option, and I got a 633 KB JPG. Can anyone help? Do I need to have icloud for Windows on my computer to download exact file size?