Yes, it's true. Nothing can be said about a compressed archive, but, given the good faith of itch.io developers, it is likely to be not a huge sequences of just zeros. However, it is also true that elaborated images often take up a lot of space, provided they do not use compression with loss of information as suggested by jpeg argument. There is also the possibility of combining digital and vector graphics while developing, without compromising on the final quality of the levels, with minimal space consumption and with the possibility of further reducing the final product through compression.