Andrea,
Its likely your history. Mathematica stores all previous results (i.e. ...Out[12], Out[13], Out[14]...) Your intermediate text results are very large. Turn off History with
$HistoryLength = 0;
or if you want to use % to refer to the previous result and not store it in a variable.
$HistoryLength = 1;
Now you may also have to clear old variables you no longer need to free up the memory.
You can see information about this in Memory Measurement and Optimization
Regards,
Neil
