You do realize that the Xbox1 used PC desktop standard sized 3.5" HDD right? And that the PS3 and 360 don't?
2.5" drives are very tiny and add very little to the overall size of the console. They are very thin too. Only about 5mm thick.
Here is a photo I just took of the PS3's HD. Next to it is a Microdrive HDD from an Ipod mini (back from when they still had HDDs). And then some coins for size comparison.
Anyway people have to get out of their heads that mass storage systems make consoles big and expensive. It's 2007 for christ's sakes. It's a totally different world from the one the XBox1 was launched in. Those parallels are not going to be relevant this gen.