I never really understood this criticism. Um, yes? Resident Evil has ALWAYS had big, giant versions of creepy things.
Sure, it's not like RE1 which had such classics as giant snake:
Giant spider:
Giant shark:
Giant... plant:
Or who could forget the classic Resident Evil 2 with its giant alligator:
Or Resident Evil 3's epic giant earthworms:
Well, it's at least not something like a weird zombie elephant right?
Okay, okay. So what about Resident Evil 4? That game didn't have any giant-sized versions of normally small things, right?
... Yeah, I really don't understand the complaints about Resident Evil Zero's enemies being "unimaginative". They were exactly in line with the type of enemies I would expect a Resident Evil game to have... though even I wouldn't have thought of mutant singing leech queen guy.
Resident Evil Zero's problem I think is less that the enemies are mostly giant animals, and more it just... Doesn't do anything interesting with all of them.
With a few exceptions (like the Leechmen in particular), most of the enemies in Zero just don't do anything interesting. The first three bosses in Zero all have boring monotonous battles (Scorpion walks a few paces, does a long animation, and then lunges. The Centipede literally just runs a track in a loop, it doesn't even go after you unless you're in its way. The bat flies around and may pick you up and calls many bat enemies), and the rest of the enemies are kind of just... There. They don't have very interesting behaviors, and there's nothing to really make them stand-out. There's notably no interesting twist on zombies in RE0, with the exception of the one that comes out of the freezer. REmake/RE2/RE3 have a number of fun twists on the basic zombie enemy through their courses.
To use the examples you named, each of those are giant animals, but have fun tiwsts and limits to how they're used:
-The Giant Snake is built up with the character of Richard, and can poison you, the first enemy who can do as such. It follows you in a creepy attic location, slithers away on defeat, and will suddenly appear and chase from behind you in its second encountered in what looks like a normal hallway before your battle with it.
-The Giant Spider in Remake (I'm talking about the boss one since it's pictured, and there's giant spiders in literally all the classic main RE games) has an arena with smaller spiders on the wall, the game introduces the flamethrower right before this for use, and you can skip the battle/make things more frantic by trying to snip away with the knife (or flamethrower) the webs blocking the door out of there.
-The Giant Shark appears in only one room in REmake/RE1. In REmake, there's smaller sharks who attack you, but there is a giant shark who will come at you
with a Jaws-like theme who can eat you in one gulp and lunges from the watery depths, super sized to you, and you can faintly see in the murkey depths as it comes for you. With this, once you've drained the Aqua Ring, you have to go by its giant carcass, and then get trapped in a corner as it comes back alive.
-Plant 42 is built up with a number of rooms and overgrowing weeds before you ever encounter it. You can beat the boss by a puzzle involving mixing chemicals, which gives you options instead of just fighting, but its fight also has unique patterns which leaves you to having to avoid all the acid going everywhere in the match.
-The Giant Alligator slowly closes in on you in an enclosed location where you either have to unload on it, use the explosives to the side craftily, or get eaten. It mainly plays on the tight corridor and limited time aspects.
-The Earthworm has a few moments you encounter it, including one where you have to press switches while it burrows after you, though the battle against it is kind of eh later in the graveyard.
-The giant statue of Salazar is hilarious.
It's my opinion that while people complain about what the enemies in RE0 are for being oversized animals since it's easier to pick out, the ACTUAL reasoning most may not know how to word well is that the encounter design with the enemies in Zero, with a few exceptions, is kind of boring and lacks the same presence seen in other games, variety, or creativity.
And I say this as someone who actually likes Zero decently. I like the locations in general, think it has pretty decent puzzles (and some of the better ones in the series), and like Rebecca/Billy.