There are 2 installations errors that might make your signature look bad in Apple Mail, both of these errors are easy to fix.
1- Do not copy your signature from Safari, use Google Chrome instead.
When adding your signature to Apple Mail, be sure not to use Safari. Safari tends to change the signature when copy pasted and it will result in larger logo, larger social media icons and the layout might also be affected. Another thing to note is to make sure you uncheck the "Always match my default font" checkbox before pasting your signature.
2- When pasting your signature in Apple mail make sure to uncheck "Always Match My Default Font before pasting your signature." before pasting in your signature.
Please be sure to check the installation instructions for Apple Mail.