PNGs also have optional compressed text metadata chunks, and it's possible to sneak a decompression bomb into one of those as well. You can get about a factor of 1000 in the compression -- 1MB of 'a' winds up being about 1040 bytes. You can have multiple itxt chunks, and it appears that the chunk size is only limited to 2^31-1.

See https://github.com/python-pillow/Pillow/blob/master/Tests/ch... for a quick way to generate some of these.