Giant spiders and scorpions are a favorite of movies, but spiders are pretty fragile. Would a colossal housecat (I’m thinking maybe tiger-sized or even bus-sized), for example, be dangerous to humans? A crow? A wombat?
Many of the modern marsupial animals had larger versions which were considered to be megafauna during the Pleistocene (giant kangaroos, wombats, echidna etc), some of them overlapping with the human occupation of Australia. Few of them are considered to have been more scary or problematic, but we did have the thylacoleo, with no modern analogues, except for the dropbears that attack tourists. It represents a genuinely scary predator to be opening yourself up to.
A cat increased in size (and appetite) several fold would be a scary thing to ponder. Stealthy, smart, agile and dominating Youtube - not to be reckoned with lightly.
All of them.
Ants. Why’d have to be ants?
I’ll take bees and wasps for $200. Some of them are already too big.
Basically, any insect.
Rats. Crocodiles. Humans. Dogs. Cats, of course! Fleas. Mosquitos. Piranhas. Rabbits. Squirrels. Dung beetles. Bears. Ticks… wouldn’t it be easier the other way around? I believe butterflies in their egg and pupal stage would not be more dagerous if enlarged, no matter how much. And giraffes, if they were bigger we could always see them coming and hide in time.
You can’t scale animals up and have them still work mechanically. A giant ant would collapse in a heap unable to move. A Guinea pig expanded to elephant size/weight would have legs narrower that a human’s. Which would break the instant the animal tried to stand. Elephant’s legs look like tree trunks for a reason; that’s how big they must be to hold the weight.
If you want to know whether a tiger-sized housecat would be dangerous, we already have those. They’re called “tigers” and they are notoriously dangerous to humans. A bus-sized housecat would be an inert lump unable to move.
Yeah, it depends on what you mean by “enlarged”. Giraffes exist, so clearly a 20-foot-tall mammal is possible, but a human even less than half that size is doomed to a very short life, and if you somehow scaled up a human to even 12 feet tall, they’d be unable to stand without passing out. A giraffe’s heart must be more than three times the size of a human’s heart, in order to pump blood all the way up to its brain.
And spiders, ants, and other tiny creepy-crawlies are strong for their weight precisely because they’re so small.
You guys have obviously not seen “Them” or “Night of the Lupus”.
Any of the weasels, ferrets, minks, etc. I read once about an escaped mink that was seen stalking a cow! Shrews.
We can certainly talk about something that might be what the OP really meant, or at least is adjacent to it. Specifically:
Gram for gram or kilo for kilo, what critters have the highest fierceness to weight ratio?
Over the years we’ve done that question to death (heh 
).
Of course for comic relief there’s also this opposite-side thread I started ~13 years ago:
Nitpick, “Lepus.”
That is all.
Dang it. It’s never lupus…
Leaping Lepus!
One of my college roommates used that as a general exclamation. Works great.
I think that’s what the OP meant by “but spiders are pretty fragile”.
Maybe. Or maybe what he meant was their legs are easy to pull off at their normal scale, so would also be easy to pull off at big scale. Hard to say.
What animals would be more dangerous if enlarged?
Frogs. South American horned frogs in particular. If you walked by a human-size frog, he’d reel you in with his sticky tongue and bite your head off quicker than you can scream mommy!
Yeah, I was handwaving all the anatomical/mechanical issues. So cats are really just tiny tigers, I guess.
But then, the opposite would be true, and scaling animals down would give them disproportionate strength to their size.
I’m not too worried about a cat that has been enlarged to the point of immobility, but a clowder of miniaturized cats would be the thing of nightmares.
Imagine a 20 tall chicken or any other flightless bird super sized. They’d be terrible.
Terrible lizards.