The only thing that was exponential was the experience from using a skill (per swing that hits, per spell cast, etc) needed to advance to each new level. But that was for the skill. It could well be a fixed number that is gained in Skyrim to reach a new level, but they have stated repeatedly how higher level skills raising will count more. What this tells me is that they may weigh the actual gain for a skill change such that it is not a gain of one for one skill up. In other words, yeah it takes say 20 total "points" towards leveling each time, but gaining one of these level up points may not be the equal of one skill leveling up once. That could get multiplied by an ever increasing amount like 1.05 at skill level 20 but 1.45 at level 40 or so on. In fact, if there is a fixed level cap, then this really strongly speaks to a direct proportion amount to the skill level itself, since then only would the series totals for all 18 leveling to 100 be the "max" level.