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.

Ah, that makes sense.

Thank you for enlightening me!

Ereaders have offline dictionaries. My Ereader has one based on Webster 1913. It works fine but it is of course missing words in modern books. But now you can get Wiktionary on many Ereaders which is just great.