Hi ! When you save a dataset, it saves the whole thing. Therefore at each iteration it writes both the data from encoded_rvl_cdip but also all the chunks of data you loaded from data until that point.
So this is not exactly what you want since at each iteration it writes a really big dataset, that is getting bigger and bigger.
What you can do instead is simply save the chunks that you want to add in separate datasets, and then load first your original dataset, then load each chunk, and then concatenate them together