Drmoo: zlib is a form of file-independent compression. I don't think it's what you're thinking of.

At the end of the day, if you take a bitmap and enlarge it ('zoom in') you can't pull the extra detail out of thin air. You either already have a higher-resolution version and fade between them (mipmapping), or you don't and hopefully use something like bicubic resampling to try to avoid pixelation.

As to the j2l file, I'm not sure what you mean by zooming in to 640x480. Bear in mind that the level described by the file could be any size, even a single tile.