The “Auto fit content” feature in Samsung Email shrinks email content to fit the screen.
In HTML emails it is common to add
width attributes to
img elements to cater for Outlook Windows desktop email clients. CSS is often used to size these responsively for all other email clients.
Samsung Email seems to check for HTML elements with
width attribute that has a value wider than the viewport to determine whether to shrink the email content regardless whether CSS is used to size the email responsively.
This is documented in more detail in Samsung not responsive when width attribute used in template · Issue #73 · hteumeuleu/email-bugs (github.com).