Impossible to say without seeing the code, but given you're talking about the train wreck of how NOT to build a website that is SCSS... First thing I'd do is open up the document inspector in each copy to see what's different. ASSUMING your target environment has a document inspector.
Seriously, SCSS is doing NOTHING for you a bit of common sense, learning to group selectors, learning to leverage your semantics (if any), etc, etc cannot already accomplish. It's a giant card-stacked lie and an "extra step for NOTHING" -- that much like HTML/CSS framework idiocy I cannot grasp how or why anyone would use them by choice.
But I guess in this age of using 100k of markup to do 10k's job and half a megabyte of CSS to do 48k or less' job, it's hardly a surprise people get suckered in by any of this nonsense.
That's all wild guesses though since we have no idea what your code is, how you chose your font-sizes, what metric those sizes are declared in, if webpack is screwing with your load order (which happens!), if your testing environment has a different system metric (default font-size) than your production, etc, etc.