Why do I cannot center elements if i scale them

I am using this to center things in CSS:

.testclass {
display: flex;
justify-content: center;
}

but when i want to scale elements using width and height, it doesn't work and my elements are not centered.

Like this:

.testclass {
display: flex;
justify-content: center;
width: 200px;
height: 200px;
}

What's the problem?