Outlook 2007/2010/2013/2016 will display images in a smaller size, even if the original image has larger dimensions. This happens because Outlook does not interpret the size of the image from the style attribute, so Outlook will only resize the image if the width and height attributes are given.
We have recently updated our Email Wizard to automatically include these two attributes, if you have resized the image using the drag functionality.
If you are using Custom HTML, we recommend using the width and height attribute for your images like below to ensure your image works correctly. <img src="http://www.mysite.com/logo.gif" height="80" width="400" border="0" alt="" />,