They are really big.
I suggest to try Orbis and of course Wildmana. Those are most balanced.
RiFE is the most fun for me, but mostly because I was a part of its creation and it has many things I like. It also has some things which totally break FFH2 lore for me, but other mods have something like that too. It's just a bit too "MOAR" for my taste, but it's good!
So try all 3 of them. I won't describe RiFE because it will obviously be biased. Just try it. It also has VERY strong barbarians and animals, to produce the "hinterland" feel and limit the expansion and preserve the wilderness, so it's not for newbies. Also there's a new patch apparently, I should definitely play that again.
Orbis tries to be very organized and to introduce interesting gameplay mechanics the most. It also has quite a cool tech tree which is different from Vanilla FFH2 (lol) and other mods, sometimes it makes even more sense, so it's good.
Master of Mana tries to be close to MoM, it has very interesting spell research system and global mana points system, also it introduces global spells (don't confuse with the "wolrd spells), enchantment, global terraforming, revamps the summon system etc. Basically it makes the magic to take a lot more place in the game and to be much more interesting. It also splits the tech tree into military/civillian/magical totally different screens, so it's revolutionary there.
And of course even a whole thread cannot describe differences and other things which are included in these mods. GO PLAY THEM!
P.S. Just tried to find my original "Malakim++" mod which started Fall Further ++ which turned into Rise from Erebus (Valkrionn is such a cool guy), but it looks like that part of the Civfanatics archive is gone
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P.P.S. God damn it, how long have I been modding games? My liiiifeeee...