This is a lovely question.
So easy to ask, and yet almost impossibe to answer without writng a 500 page book! I'll try to make a start!
Let's consider a Giraffe and a Beetle.
They are different, yes? No doubts. They are just so different. And they are classified differently in science as well, a long way away from each other. Giraffes have live young, beetles not. One has an internal skeleton, the other, a hard exo skeleton. One has 4 legs the other 6. It goes on...
Now consider a Elephant and a mouse.
Different - yes! But how different? Both have live young, both have hair, both have 4 legs and live on land. Sure, there are some vry big differences between them, but if you think about the number of things they also have in common, maybe not as far apart as the Giraffe and the Beetle.
Now, fish! Okay, a Tetra and a Barb.
Tetras tend to have a small fin on their back, between the main fin and the tail, barbs don't. Look at your fish, you'll see this. There are other differences too. Tetras are Characins, they have teeth in their jaws, with small Tetras, it is not easy to see, but a Piranha is a Characin as well, and nobody doubts the teeth there! Barbs do not have teeth!
The closer you look at an animal, the more differences you can see.
Some of the obvious differences like size, are really not that important. Think about dogs, a Labrador is bigger then a Terrier, but they are both "dogs" and will breed with each other.
Other differences are more difficult to see. The differences between a Barb and a Tetra maybe difficult to see, (they swim, they are the same size, they have a tail...), but they are very different in close up, in detail, and it is the detail that matters.
A Labrador and a Terrier are very closely related, a Barb and a Tetra are a lot further apart.
Don't worry about size. A Neon is closely related to a fearsome Piranha!
A Cherry Barb is just as closely related to the mighty Mahseer!
... but they are different from each other by just as many features.
Ah, there is no way I can explain this without getting really technical!!!! I tried!!!!
