Why is it so big?

One of the reasons has been addressed in the past github.com/mozilla-mobile/firefox-ios/issues/32679 however that still doesn't work for some users. Unfortunately inspecting the individual databases means access to personal data so it's hard to do remotely — you might offer to share some of it privately with the engineers on that ticket, to inspect whether it's the same Suggest database, or something else — or perhaps ask for instructions how to e.g. export the databases locally for you to report at least their individual sizes per storage type to figure out some next steps what to look into.