>Thats done the job - it's shrunk from 100Mb to 86Kb - How on Earth did it get that big in the first place?

I think the biggest reason is changing view definitions or configurations. These are stored in memo fields and each time you make a modification, the memo file will grows because it takes what you just modified and appends it to the end of the memo file and resets the address in the table. The information that was previously stored there is still there, just in a location that is not referenced anywhere.