By default, any Button layers in your designs will be set to be fluid on mobile and expand to the width of the Column (depending on the size of the mobile device screen viewing the email).

You can change this behavior on mobile by clicking on your Button layer, then clicking the Settings layer in the plugin header, and toggling the Full Width Button on Mobile toggle option on or off.

If the width of your Button in the Figma (desktop) design exceeds 300px, the button will automatically be exported as Fluid, otherwise the longer fixed width button would push out the side of the layout on smaller screens.