BrandMail email signatures are generated as high resolution images. The display of your signature may look different depending on the device you are viewing your emails on. For example, external monitors are generally set to display at  a lower resolution than say, your laptop. 

If you are using BrandMail desktop, there are a few settings in Outlook and Word that affect how images are handled by Microsoft Office. 

In Outlook go to File > Options > Mail

Click on on Editor Options and then Advanced

Scroll down to Image Size and Quality. Adjust as required. Typically a resolution of 220ppi is sufficient. 

Also see: My signature size keeps changing for related troubleshooting.