Wiktionary dumps are around 1gb for a million words and definitions (https://dumps.wikimedia.org/enwiktionary/) uncompressed. However this is just raw text.

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.