Sauce Labs has little to no control over how a customer's site is presented on our devices. When a site appears with a desktop layout on mobile, or vice versa, the cause is almost always a user-side issue.
Here are the most common causes for incorrect layout:
Incorrect User Agent Sniffing
A User Agent is a header automatically sent from your browser, telling the server what kind of browser and operating system it's for. User agents are pre-set by the browser manufacturer, and Sauce Labs doesn't make any alterations to this value. Trying to guess what kind of page layout to send to a browser based on its user agent is called User Agent Sniffing, and it can be somewhat unreliable. See This KB Article for an analogy that explains the problem with user agent sniffing, and some suggestions for alternatives
Automatically adjusting page layout to match the screen size is an example of Responsive Design. If you are using a Sauce Labs browser or device whose resolution or screen-size differs from your local testing environment, the difference may be enough to trigger your site to display differently.
To avoid this, use the screenResolution desired capability to select the right desktop resolution, or use a virtual or real device with a closer matching resolution.