What is ROM compression?
Rom compression is a feature of the EZ3 that allows you to compress the ROMs down below their original size giving you more space to put things on the NAND area of the cart. The downside to this is that it takes time to decompress your file and so more time to get to your game. As a better method of compression (one that results in a smaller file) would consume more cpu power (which the GBA lacks a lot of) the compression you will see is not as much as you are likely to see in a ZIP/RAR file or other compression methods you may use.