theme.json has a “layout” setting for content width, but that would not only affect previews. Most themes output a viewport meta tag, but there’s not a specific theme setting for it, it’s almost always width=device-width. Again, would not only affect previews.
I suspect there’s an odd editor style limiting width. Use your browser’s element inspector tool to find what CSS rule is limiting width. When found, the tool will also tell you where it comes from. If you have CSS optimization in effect, it will obfuscate the rule’s source. The optimization should be disabled for this investigation.
