I think 9-12 is kinda stacked too and never feel like I run out of quests to do in that range because many of them in that range are fun, fast and yield decent experience. Ravenloft is such a godsend really, that expansion to this date is just carrying.
Meanwhile, 25-29 is hurting way worse than any other part of the game by far, partially because there are so few actually level appropriate quests that aren't at least partially something you may or may not have done in heroics. So few quests are specifically there and intended for that level. Giving heroic/legendary quests the chance to be played at an Epic difficulty would help a little bit in terms of first time experience yields, but not in the way where people who like doing quests instead of slayers tend to run out of things to do between 25 and 29 unless they start dabbling in Legendary content, repeat content or do underleveled quests - or a combination of those.
I'll refer to my spreadsheet where I've logged XP totals. I'm also going off repeated runs through many lives; you don't want to run the same content constantly so more depth is needed. L3 to L8 is good, even the slightly weaker levels like 4 and 6 aren't bad, and there's enough surrounding them at other levels that you can bank or run under level and be fine.
Level 9 and 12 have 19 quests, 20 for 11 and 22 for 10. 5-8 has 30+ each, as does 3 with 37, the third densest level. 1, 2 and 4 are lighter, but 1-2 doesn't really need a huge amount of XP, and 4 is at 28 with the new pack, so not far off of the rest, and surrounded by the glut of L3 and L5.
Then there's the actual quests:
L9 has Threnal, which is both terrible and bad XP and bad loot. Some of which don't even give favor. Sorrow Dusk is gated by lower level spread, as is Shadow Crypt. It is helped by the massive XP of Shadow Crypt, VoN 3 and 4, and White Plume at least, as well as L8 having both a glut of 30 quests and some excellent XP ones and the IoD saga, which follows L7 which is another glut. Red Fens at least gives decent loot to upgrade to L20, if not spectacular, but isn't particularly great XP outside the last one.
L10 has more Threnal quests which are terrible, just slightly better XP if you hate yourself enough to run them, more gated Sorrow Dusk which are fun but mediocre XP (and very obsolete loot) but the biggest chunk of XP is gated in VoN 5 pre raid, and Tempest Spine, with another chunk in Ravenloft 1. Ataraxia is okay XP, as is Newcomers, but L10 is more practically 20 quests for short groups or soloers.
L11 has most of it's XP in Ravenloft 2, Offering of Blood, and a couple free quests. And Necro 3, which is horrible and a cancer on the game. Necro 3 is why there's not 10 million players for DDO. The devs would be billionaires if not for Necro 3. There would be a SSG/Turbine cult if not for Necro 3. The bulk of the rest is poor XP Desert and free stuff. L11 definitely needs some help.
L12 you can argue that it doesn't need anything; RL3 is massive XP, Eyes is good XP, the bulk of the great Desert XP is at 12 as well. Relic and Invaders isn't bad either, and you have the RL saga to make up for the short fall of 11. Still, it's tied for third least amount of quests in Heroics.
No arguing that the devs have done a very poor job on building the Epic quarter of the game. There's 77 quests at L32; over twice what is available through L25 to L29. Outside of L21 with 27, none of the Epic levels are good for the number of quests. Most of the Epic quests are old content as well, with the loot being horribly obsolete.
Will someone fix the forums? No other forum I've ever been on is so poor about formatting, and removing white space.