4.4 pulley size wrong

[Ryoken]

I got a 2002 4.4 with a bit of a puzzler for you guys,
I got a squeak from the engine ,  took car up to local indie and he said it was an idler pulley , gave me the part number and I bought one. dropped it in today to get rid of the squeak sand he brings it back saying its the wrong size.
"Steve
We have returned the beast although it's not fixed the pulley supplied is 2 large I have check on all BMW parts catalogues and both pulleys should be 80mm which is what has been supplied although the ones on your car are both 70mm I am  going to have to do some digging over the next couple of days and find out what's going on
I'll be in touch"
any one got any ideas why mine have got wrong size pulleys on?
Cheers

Steve