Xbox One is thick due to various important reasons. It has a lot of power and a great cooling system so it doesn’t surprise me that Microsoft made it so thick.
The Xbox One fan needs to be larger than a laptop fan so that it can move more air at slower speeds. It is designed to keep fan noise as quite as possible. The Xbox 360 was criticized for loud fan noises, so Microsoft ensured this doesn’t happen in a newer console.
Microsoft touted the Xbox One’s “practically silent operation” in the announcement presentation and here in this official Xbox article: