I just powered-up my first Dragonite to 2750-CP. The IV is 82%. The appraisal says his best attribute is HP. But his new HP is only 135. My 2750-CP Gyarados has 150-HP. And a 2000-CP Arcanine has 135-HP.
Is a 135-HP a reasonable value for 2750-CP Dragonite?
Why are they called "tanky"?
Before power-up: 1983-CP / 116-HP
After power-up: 2750-CP / 135-HP
The appraisal did say his best attribute was HP...
The question body doesn't really fit the title, so I'm going to answer both separately.
During the major CP rebalance in November 2016, Dragonite's HP actually stayed exactly the same, while its Attack was buffed and Defense slightly nerfed. Since then, there have not been any changes to any Pokemon's stats, except for Lapras, which was excluded in November 2016 and retroactively nerfed in February 2017 to match the stat calculation formula every other Pokemon has been using since November 2016.
Dragonite has medium HP, there are several Pokemon that have more. Comparing Pokemon via CP is not very useful, though, since their stats can vary greatly. Dragonite has higher Attack and Defense than, say, Arcanine, which is why your Arcanine with the same HP has lower CP. The best way to compare Pokemon is by their level, which is the position of the little dot on the arc above the Pokemon's sprite. If it's all the way on the right, the level is your Trainer level +1.5, so if 2 Pokemon are both maxed out and have the same CP, their total base stats and IVs are roughly the same.
As for why Dragonite is called tanky, this is mainly due to 2 aspects. First, its Defense is rather high, especially in comparison to other Pokemon. This reduces the damage it takes, making it live longer even though its HP is relatively low. Second, its typing gives it several advantages over commonly used and strong attackers. Dragonite is weak only to Fairy, Dragon, Rock and Ice, all of which are hard to come by on very high CP Pokemon (Ice being the only one that's realistically available on high CP attackers at all). It resists Fire, Grass and Water, among others, which are some of the most common types of high CP attackers' moves. With the prevalence of Water-types in gyms, Grass and Electric are preferred choices for attackers, and a Dragonite thrown into the mix will only take neutral damage from Electric, while it doubly resists Grass, making it hard to efficiently take down a Dragonite in a Water-dominated gym. Gyarados, for comparison, takes neutral damage from Grass and is doubly weak to Electric, so even though its HP is a little higher than Dragonite's (which is also offset by its slightly lower Defense), its typing makes it much easier to take down with the attackers people are using anyway.
