I'm writing my fix if other people will encounter the same problem. Also, that I must stress that this fix will work only for Cyangenmod users!
The problem is that there is not enough space in /cache/, so when the market wants to download a "too large app", it throws an exception saying "there is not enough free space". You can see that error if you have the android SDK and you run "adb logcat", and try to download an app (you will get "not enough free space in internal download storage").
So how to fix it? The easiest way I found is to download an app called "S2E". You can try to download it from the market. If you can't, look for the APK, and install it from the SDCard.
Once you installed it, you can choose directories that will be moved to the sd-ext instead of the phone's internal memory. Check "Download cache Location: /sd-ext/download". Than click on "menu" and than "Reboot".
That is it! :-)