I am using ffmpeg to convert an audio stream to ac3

Now I have used this utility before with great success to give me small but good quality files, now something has gone a bit strange

Here what occurs:

I took a multiplexed file of 47.5MB and ran it through ffmepg to get an AC3 conversion of the audio stream and ffmpeg gave me a MASSIVE 11.7MB AC3, I also tried it with the MP2 output and got a MASSIVE 20MB audio file

Now I then ran the ORIGINAL multiplexed filed through Tmpgenc to demulitiplex it and the original audio file (mp2) is only 1.98MB

So what is going on? (BTW I have tried it with another file and the same result of a MASSIVE file)

Could I have downloaded something earlier today that has ballsed up the compressing 'bits and bobs' (hey Im a newbie LOL), the reason I ask is that not only am I having probs with ffmeg but I also tried to run a 150MB file through IFOedit and the thing came out as 9Gigs big WTF????

Earlier I downloaded ffdshow, wax 2.0 video editor, steeper 11 and virtuadub just to give you the details, could one of these be causing the problem?

As I say I have used ffmpeg before and not encountered this, so what gives?