The enlargement or resizing of images is almost always because the original image is uploaded or the embedded image in the initial email is way too large. Although it's resized in the editor with overriding height/width settings, the recipient's email program will sometimes strip those instructions only to display the originally sized image (you can't control what the recipient's email program does). 

The only way to fix this is to embed the right-sized image in the email initially. You can do this on a number of websites, but this one at Adobe is free and easy: https://express.adobe.com/tools/image-resize.

Typically for an email you would want to set the width somewhere around 200-400px (pixels). It's worth noting that often the recipient receives the email with a smaller image.  It's the "reply" that strips the formatting.