OK, thank you.

That makes sense now. Will that growth be permanent after the rebuild has completed or will the database reduce in size again once the rebuild has completed?

I assume then from your reply that turning the sort results in tempdb on will stop this happening? Will that then cause the tempdb to permanently increase in size?

Sorry for all the questions, this is all new stuff to me so the thirst for knowledge is quite high (at the moment :-D)