File differential not display properly when committing via API

Hi everyone!
I’m using the following API documentation to create commits:

I’ve noticed that when I submit a commit via the API, and I look at the differential between the two files, the entire file is regarded as changed (completely red), even though I’m only adding a few lines. Why would this be?

For reference, I’m committing a rather large piece of XML via a PowerShell script that I wrote. PowerShell is formatting the JSON needed to POST to the API.