I think student will prefer larger laptops like 15", while working adults would like smaller laptop for those on the run meeting clients.
Bigger laptops for students like insprion 15 7000 is made to cater for this group. Bigger case can pack bigger specs and use normal cooling. It is a value proposition for students.
For working adults, they may prefer XPS15 or XPS13, for the lighter and professional look to impress clients. And because they are on the run, and with documents in the bag, a lighter laptop is useful. The XPS will be more expensive specs for specs, but they have more ram and ssd as default, good for storing work files and simple media, that loads up faster
I think that is how Dell do their research and come out with insprion and XPS brands for different segment