That's pretty disappointing that MacOS has an unmatched dictionnary app but iOS is forgotten while there is decent options on Android.
I believe it works offline, too, since the device I use for the Times is almost never hooked up to the internet.
I'd recommend wiktionary.org. Although it is an online dictionary it's part of the Wikipedia family and hence are better compared to other companies with regards to privacy.
I spend around 500 megabytes on offline maps for OsmAnd and don't really worry about it. I would have naively guessed a million word definitions wouldn't be larger than that.
Why do you say it's too large for fully offline usage?
We have images as well like https://www.vedaist.com/en/w/clarsach.html. Audio pronunciations in wiktionary are sound files. Binary resources take a lot more space.
For mobile apps, if the size increases a lot then the first download experience is not great. After a certain size the app is not usable for low end smartphones.
Hence I'm not sure about fully offline for a rich/modern dictionary mobile app. Downloading partial information offline like a temporary cache is possible though.
Thank you for enlightening me!
The mac default dictionary is great for offline use and passive learning. With vedaist features are geared towards active learning with gamification.
Disclosure; I'm the creator of the Vedaist project.