7xxx is a workstation, so of course it's heavy if you compare it to ultrabooks. It is effectively a portable PC, so I don't understand your point. 7xxx is not "very similar" to XPS, it's a completely different chassis. 5xxx line however shares the same chassis with XPS - it's thin and light, but it's thermals are not even in the same league as 7xxx. So to summarize, if you want a beast of a workstation - get the 7xxx, if you want an ultrabook - it's 5xxx or it's cheaper, watered down version, the XPS.