I would have though a few tens of kilobytes would be all you would need to store the list of words.
I went back to look at
https://whatfreewords.org/about.html to see how many words were in use and it's gone. Lawyers I presume.
But if we take 40,000 words as a given:-
40,000 words * average 6 letters per word = 240,000 bytes.
Compression of english text (with no words repeated) is going to be hard to get better than 50% so you're going to have 120,000 bytes minimum for storage of the words. That's way more than "a few tens of kilobytes" but still way less than 10MB.
If they ship one database with multiple languages then I can see how they can get to 10MB for the database.
Mind I'm not sure where this DB sits as I believe the translation between Lat, lon and the three words take place on their servers. So the app doesn't really need a copy.
The whole point is that the app works where you have no data connection, so the app must have everything it needs to do the conversion.