Are you sure they are compressed?
Usually they are zipped, and then decompress to .bin files. A .bin file isn't necessarily compressed; it might just be a generic binary file. It would be openable with Stella in this format.
Here's what I did to access some .giga files in a .bin file:
1) Select both the .bin file and the corresponding .cue file
2) With the cursor on the .cue file, right click and choose "Open with Toast"
3) Change the Destination to "Disc Image"
4) Click the Save button and name and locate the resultant .dmg file as you wish.
5) Under the Toast menu item "Utilities" select "Mount Disc Image" and navigate to the .dmg file youjust made. You should get a generic disk drive ico mounted on your dektop.
6) Navigate to the .bin INSIDE this mounted disk, and again select "Mount Disc Image." This time you will get a CD icon and a message saying "This disc is unreadable etc..." Select ignore and access the content within the disc image.
If you have no luck with decompression tools available within Lion or Snow Leopard, you might want to consider that either those Atari ROMS are simply not decompressible with or without existing tools, or that perhaps they were originally compressed using something else.
After I read your post, I thought maybe there is a functional Atari emulator for OS X that would have tools to deal with those Atari ROMS. That lead me to the Hatari emulator sources. Among the pre-requsites for building Hatari, was also obtaining and building Zlib from Github sources. Apparently, Zlib is its own compression/decompression animal that appears to used for .png data compression in libpng. It apparently will also deflate compressed data within PDF files.
It is a stretch to presume that Atari ROMS may be compressed with Zlib, but since nothing else may have worked, it might be possible that the Zlib library could be the can opener you need. It may be worth reading further.
On the Zlib site, they have a link to a heavily documented stream I/O source file called zpipe.c. There is a downloadable copy of it. You do not need to download the Zlib library code or anything else. Just get this zpipe.c file. I have Mountain Lion and the zlib include file and library are already included in the OS, as they are in Lion. I have not found any Zlib command line tools in either OS X release though.
I have Xcode 4.5.2 and command-line tools installed in ML. You won't need Xcode itself, just the command-line GNU C++ compiler. You need to open zpipe.c in the editor of your choice and change the following line only:
#include "zlib.h" #include <zlib.h>
Now, save the file.
Compile it in the Terminal (on Mountain Lion 10.8.2, the library file name is libz.1.2.5.dylib)
g++ -O2 -o zpipe zpipe.c /usr/lib/libz.1.1.3.dylib
I got a clean compile on Mountain Lion with the 1.2.5 version.
I haven't been amazed in awhile, so if this works, it will be worth it.
You would use zpipe in the following manner from the terminal to possibly deflate [ -d ] a COPY of an Atari ROM file.
zpipe [ -d ] < Atari ROM file > Dest File
I have over 700 Atari ROM files, and they all have .bin extensions. They all work fine as-is with Stella. The .bin extension stands for binanry, and there are a lot of programs that uses binary files. I suggest that you try to open one of these files with an Atari emulator (like Stella). Also note that the icon look like compressed files because they open in Archive Utility by defualt (double click). You can change that in the Get Info pane.