The growth of the size of an arc is only misleading because the graph naturally draws you to compare different browsers as a percentage of the market.
Since the market has grown basically linearly, the arcs may actually be in direct proportion to the actual number of users, in which case there is very real growth.
The confusion comes when you see growth of absolute terms when the more interesting statistic is what the relative growth is.
Particularly, if one browser's arc remained the same size on each ring you might assume that it is staying the same. It has the same number of users, but it is losing prominence.
Since the market has grown basically linearly, the arcs may actually be in direct proportion to the actual number of users, in which case there is very real growth.
The confusion comes when you see growth of absolute terms when the more interesting statistic is what the relative growth is.
Particularly, if one browser's arc remained the same size on each ring you might assume that it is staying the same. It has the same number of users, but it is losing prominence.