it's the alcohol. lots of (single) ipa have significantly higher hop bills than double and triple. the original gravity and yeast sets the ABV. many doubles are smoother in mouthfeel than singles, although run the risk of having a boozy aroma.

and bitterness is related to hop content, but specifically alpha-acid hop additions, so it can be controlled independently of the aroma hops.

secondary fermentation is common in many styles, other than ipa, and doesn't typically have a huge influence on alcohol content, but rather allows flavors to mature, or become more pronounced. more yeast falls out of solution, for a clearer beer, and it can naturally carbonate.

some yeast strains will cause extra fermentation as time goes on, including when it's bottled or aged in a cask, but that is not the case for most commercially available ipa.

and if you really want a smooth, drinkable, high alcohol beer, try finding a Baltic porter!

[–] anarchiddy@lemmy.dbzer0.com 1 points 6 days ago

I'm not saying it's the quantity of hops, i'm saying it's the hop-forward and fragrant flavor characteristic of IPA's. You said it yourself - there are plenty of other high ABV content beers that aren't as bitter or hoppy as single, double or triple IPA's. More than that - there are plenty of straight IPA's that have the same or similar ABV as some double or triple varieties.

High ABV often comes with the double or triple label -true enough- but the origin of those beers wasn't a desire for a higher alcohol content but (in true American form) a doubling-down on the unique flavor of the IPA, characterized by the bitter hop-forward profile of the historic India Pale Ales (which were made to survive longer trade routes from England without spoiling originally). The Triple IPA designation is more recent and less common and does get cited as an option for higher ABV content by some brewers, but that's not the case for most double IPA's and it's more of a recent trend that I wouldn't say is typical.

if you really want a smooth, drinkable, high alcohol beer, try finding a Baltic porter!

Yea no argument here, plenty of other beers with enough kick to end your day early without the bitter aftertaste