Probably so it's more likely run out of oil causing huge expense for unwary owner and more profit for the motor trade.

A few years ago I checked the oil level of a small nearly-new Citroen with a mere 4,000 miles on the clock [the young owner was about to take one of my children on a long journey] and it didn't even reach the end of the dip-stick! No problem for a while if big capacity sump, but recipe for disaster if small reserve.

It's strange as the diesel model has a 5 litre sump but the petrol makes do with a 2.7. However, it must be using oil at a rate of a litre every 5000 miles, which is nothing really to worry about, but I will check more often as such a sump is too small. However, unless as you say, it's some scam to make more money, in which case Nissan shoud go for a standard 5 litre sump.

Why make it any bigger?

Several reasons realy, 1) So you have more oil to even out the pounding that the oil gets being pumped around the bearings etc, so in effect the oil get worn out less quickly 2) More margin of error for engines which burn a bit of oil or do not get checked very regularly 3) More oil means oil more likely remains slightly cooler. 4) I'd rather have more oil than less in my engine.

Surely a low oil level is a low oil level, whether the sump holds 5 ltrs or 2.7 ltrs,

My 05 1.8 Almera doesn't use any oil between services and a 2.7 ltr sump just goes to show what a well designed and engineered engine it is IMO.