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!