They don’t, it’s a proxy for size of luggage. They want to encourage people to pass through the airport (and onboard) unencumbered by large luggage. Only when they describe in terms of size, people argue over dimensions and ‘squishiness’ and so on. If they say ‘wheels=suitcase’ it’s very clear. I personally think the Turkish authorities deserve credit for trying to make the process easier, and wish others would follow. It’s certainly much easier and quicker to board without half the passengers standing in the aisle near my seat huffing and puffing and faffing around trying to get things out of their suitcase before struggling to lift it up into the lockers.