Issue
When using the Desktop version of Outlook, images in the email signature appear much larger compared to images displayed in FYI or using the web version of Outlook.
Cause
Some email applications ignore the set height and width dimensions for email signatures and instead display the image at their native size i.e. the resolution when the image was uploaded.
As the image is large in size, Outlook displays the full-sized image.
Solution
Reduce the image size in the default signature, then create a test email to confirm the results.
Resizing images can be completed using regular image editing software, for example, Photos in Windows 11.