Considering there are two substantially different versions of the second half of the game, I think the shorter length per playthrough is understandable and personally I have no problem with the game´s length per se.
I also think chapter 3 wraps things up nicely story/character-wise, especially given the amount of possibilities how it can play out.
Maybe a part of the problem is the "chapter" label which creates mislead expectations?
That said, I feel the two main branches are not fully equal in quality - I think Iorveth path provides overall more even, fulfilling and better paced experience. It has a bit more main story content, more side quests, more fleshed out environments and also provides players with more choices and gameplay diversity.
I´m pretty sure the game is longer when going this route.
What I think is somewhat problematic in relation to the game´s length is item and character progression - "pacing" and balancing of these two elements doesn´t account for the pacing of story elements and the game´s overall length too well.
Best items are usually acquired near the end of a chapter only to be quickly replaced by better ones available ïn the beginning of the next one (draug armor is particularly hilarious). Chapter 3 is worst at this because on one hand it overflows with new items which player can barely put to use and on the other hand it more-or-less makes previous gear development moot.
Missed opportunity in this regard are the mutagens and, maybe to a lesser degree, armor/weapon enhancers.
In the case of mutagens, the fact they can´t be replaced once put in an ability slot renders lesser and normal variants useful only as means of additional income. Allowing players to replace lesser ones with normal/greater ones of the same type, putting some unique ones in the world and decreasing the overall amount of their drops would work better I think.
Similarly, allowing players to increase the amount of enhancement slots on gear pieces might´ve been a good idea.
Basically, lesser amount of equipment pieces on one hand, but more customizability of pieces themselves on the other might result in an equipment progression curve more fitting to the game´s length and even Geralt´s character.
As for the pacing of skill progression, to me it felt good up till the ending stages of the first chapter, but in retrospect such pace would only be sustainable if the game was longer. But since it isn´t, final quests of ch1 provide players with huge amount of experience points, making them jump 6 levels or so higher very fast.
Something similar occurs at the end of ch2.
I think it was an attempt to fit 35 available character levels into available playtime and prevent players to get too powerful early, but as a result character progression feels too tightly controlled and uneven.
Better solution imo would be to make smaller difference between experience gained from side and main quests (this might make optional content crucial on higher difficulties, but personally that´s how I like it) to smoothen the progression curve and couple it with nerfing of some of the more powerful abilities to avoid players´ power getting potentially way out of whack too early - this could also be augmented by making mutagens more powerful or making some applicable only to specific abilities and making these only available in the later stages of the game, or something.
To reiterate, to me the game is of satisfying length from the story standpoint, but some other elements aren´t all that well adjusted to it.
Anyway, the game seems to be relatively easily moddable so maybe in time "mods will fix it", but I hope CDProjekt will attempt to make some rebalance in later patches or in an expansion too.
Sorry for being rather tangential in some bits.