I'm not quite sure I understand, are they not just part of some standard Jaguar BBS split rims? I don't think they're designed to make the wheel look bigger?!
And some cars need big wheels, following that example, would you put 14" steels on a 440bhp jag?
I understand the Up! doesn't exactly need them, but some find them aesthetically pleasing. Personally, I think 16" is as big as you need go, and I'm fact, I'd plug for some wide 15s and a good decking!
It is just a question of taste methinks. I don't believe there will be much difference between 16" and 17" on the Up, which has quite soft suspension settings. Go from 14" to 17" though and you will notice a big difference. I actually prefer the ride quality of my car on 14", 70 profile tyres (winter boots) - they may not have the same visual appeal as 17" though.