At a quick glance you avatar looks a bit like a large mosquito with a pointy beak for bloodsucking and upraised wings for flying. Perhaps a coincidence; perhaps not.
Okay, foot long hummingbirds. Consider one of them flying into your face at 30 MPH.
I wouldn’t go outside.
Especially since hummingbirds are fearless creatures that dive-bomb intruders of their own kind.
Dogfights between 30-pound hummingbirds competing for territory would be scary.
It’s like no-one has seen The Blob. This is my answer too.
The Australian magpie is not a ‘cheery chappie’ like the English or American magpie. It’s one mean dive-bombing bastard with evil eyes and a penchant for attacking humans from behind - beak and claws fully deployed.
One of those scaled up to the size of an albatross would be moderately terrifying.
Probably this:
An army of 600,000,000 ten-foot tall house-cats would would put a whoppin’ on the human race. They’d come out at dawn and dusk (crepuscular), burst into our homes, and gobble us up like mice scurrying in a barn.
Cats are mammals. Mammals have a tactical advantage over non-mammals due to their intelligence and warm bloodlessness. Physically, cats are the Arnold Schwarzenegger-Chuck Norris-Simone Biles hybrids of the mammal world.
Cat reflexes are ~10-15 times faster than humans (probably the fastest of all land mammals). They can jump ~6 times their body length—vertically. They are one of the few mammals that can activate 100% of their muscles simultaneously. Their collapsible clavicles and flexible spines allow them to squeeze through spaces little larger than their heads, and achieve extraordinary acrobatic maneuvers. Their intelligence is on par with that of dogs, but certainly more conniving. They have strong fangs (their larger jaguar cousins have a bone-crushing bite force of 1,500 psi) and 20 razor-sharp claws with which to eviscerate you. Their world population is currently ~600 million—and growing.
You better stock up with barrels of catnip before cats get bigger and take over the world.
I would expect the winner to be Australian. Everything there wants to kill you anyway.
House cats only have eighteen claws.
Unless it’s a catbus.
(If my three were enlarged they’d just be bigger tripping hazards)
I’d say an elephant, because an enlarged elephant would be HUGE.
THEM! is what I thought
Well, polydactyl cats can have as many as 30, so I averaged it at 20 
. And, I should add that house cats are not really domesticated, they are semi-domesticated at best and can survive quite well in the wild without human caregivers, unlike domestic dogs. And they have quickly evolved a vocalization frequency (embedded within their purrs) that closely approximates that of human babies, which humans subconsciously respond to (conniving critters indeed). Cats talk more to humans than to other cats. They are planning ahead. 
I recall a cat expert saying “dogs think humans are gods. Cats think they are gods.” As a servant of 6 cats, I agree.
Albatrosses have been known to attack shipwrecked sailors, so a much enlarged edition would be fearsome.
Worse yet, picture a giant petrel (which attacks and kills albatrosses) several times bigger.
“Giant Petrels are prime examples of opportunistic feeders eating almost anything they can get their bills into. Unlike other procellarids Petrels will also feed on land, mainly meaning carrion.”
“They are known to attack other birds, either beating them to death or drowning them. This applies especially to juvenile birds and chicks.”
Not nice to humans either.
“…there was one thing the giant petrel was equally certain of: My toes were meat.”
“The seabird was approximately the size of a schnauzer with the head of a pterodactyl and cold, pale, undead eyes. Grounded from flight by a recent molt, it chased me at a surprisingly quick waddle around a chain-link cage on webbed feet bigger than my palms.”
I have a 15 lb. half-Maine Coon cat, semi-feral (young, origins unknown, maybe she has PTSD). She once spotted an animal on our deck, lunged for it while growling, and I put my leg out to deter her in order to shut the screen door. So she attacked my leg in a blind fury, yowling and hissing, trying to get at the critter on the deck. I had several bleeding gashes and my leg was black and blue from thigh to ankle. This was a 2 year old house cat. I am still wary of her…
The images upthread of giant isopods are disturbing. I had a look online and came across this video of Vietnamese street food in which they gut, prepare and cook a giant isopod as part of a fried-rice dish. It’s pretty nasty in the beginning, as the isopod is gutted fresh out of the water and they pull out and discard all kinds of disgusting stuff. It’s not as bad after they steam it and scrape the meat out of the shell, and it actually looks like a tasty dish by the end of the video.
This happened not far from me, and they were just plain otters. I’ve seen the graphic photos of her injuries…she was probably only a minute away from being the first recorded person killed by an otter.
One of those 5-foot Amazonian otters, if of similar attitude, would be a nightmare.
That’s a lot of work preparing something that I’m sure–like lobster and crab–tastes like nothing besides whatever you dip it in.
I came here to post that, but you said it better than I could.
That would depend on the mode of enlargement. If you enlarged an animal with a fantasy type enlarger, like a reverse Honey I Shrank the Kids device, then yes, its size would be out of wack with respect to it’s mass. Its size would increase by a power of 2, while it’s mass increased by a power of 3. It would have trouble supporting its weight in that case.
But, if the animal was enlarged by accelerated natural or artificial evolution (not too accelerated of course), it’s proportions and mass should enlarge in lockstep, and hence it should be able to support it’s weight.
We see many examples of this in the real world all the time: some animals within the same family are quite tiny, while others grow to huge proportions.
There are of course limits to the sizes different clades of animals can attain even with the slow process of evolution. You won’t find elephant-size insects (thank goodness), or blue whale-size terrestrial mammals, for example. Maybe we will find them on low gravity, hyper-oxygenated planets, but not on Earth. Those are planets I’ve taken off my bucket list to visit.