yeah... but they don't say that when you compress a program the memory model chances and becomes "fixed" (this is also true for any other compressor though). This means for example that memory can't be "moved" anymore when it otherwise could (and thus freeing some memory blocks) or that it can't be "shared" across multiple instances (each instance would have its own memory block opposed to a shared memory block)...
...something in that order... as I said, I can't explain it very well, google can do better
/ 
