Well, for me structure is more flat if there are more paid positions with less winnings. And also if there are smaller pay jumps.
Like double or nothing is super flat.
WTA is super not flat.
Last cardschat freeroll on FTP had 67 entrants, 15 places paid, and winner took about 3.5x more then 5th place.
freeroll had 44 entrants, only 5 places paid, winner took 9x more then 5th
So the FTP has more flat structure. I suppose that is more popular for casual players, makes then cashing more often = more happy.