A 100MB update might be 100MB of new content, so your 500MB game install grows to 600MB.

Or it might be 100MB of files that replace existing files, so your 500MB install might stay the same or even shrink, if the new files are smaller than the ones they're replacing.

Or the update might itself be an application which plays a video while it's installing the update. So the 100MB update might be 90MB of installer code and video, and 10MB of actual content (which again, could be either additional files, or replacement files).

Or maybe the update is compressed, so the 100MB update actually expands into 200MB of files (so if it's all new content, your game now takes 700MB on disk).

Most likely the update is some combination of those things. So it's impossible to predict what the new size of the game will be on disk. It depends on what that particular update contains, which is in turn a function of what the development priorities were.