This is a small growth for good performance improvements (which we might see on Fuchsia as well). Feel free to use the `Fuchsia-Binary-Size` footer and explain this is a performance improvement, so the size growth can be ignored.
As for the growth, the compression algorithm pushes the compressed code into pages of 8k. This means that if the compression without your change caused an 8k page to only be partially filled, and your change causes a small, but meaningful growth in the compression that pushes the size beyond that 8k page, you will see at least a +8k increase in size.
I am curious as to why you saw +16k (meaning a whole other page was filled, which shouldn't be the case for a 4k change). I will forward this case to the owners of the compression tool to investigate.