Our team has found what appears to be an issue in the OSX version of FileMaker Server 17 (and potentially earlier versions, untested).
We have taken a copy of our development file which is normally hosted on an OSX environment, and placed it onto a Microsoft Azure windows environment. Loading the same layout with the exact same record data, we are finding that in OSX, the css file generated for "un-styled" objects is reported by Chrome to be ~2.9MB on this particular layout.
On Windows the same CSS file is reported as 165kb, with a corresponding decrease in load time.
Upon inspection of the actual css files, they do appear to be identical, and outside of performance impacts there is no functional difference between the two.
The same effect is seen on the css files generated for themed/styled objects, but less pronounced as the overall filesizes are much lower by comparison. We've also confirmed that this is not specific to the one OSX instance, as we've replicated the results across multiple OSX installations.