Rotation of object in opengl

Hey guys, iam very new to opengl coding, and i have problem with rotating my simply generated triangle. When i rotate triangle, it deforms its size(base of a triangle gets narrower, when its rotated for 90.0 or -90.0 degrees) also when i have width and height of screen bigger(for example 1400x1000), triangle rotates slower, then when screen is smaller(for example 800x600), its rotates quicker. Can somobedoy help me out?

Rotating code:

float vertices[] = {
-0.5f,  0.5f, // Top-left
 0.5f,  0.5f, // Top-right
 0.0f, -0.0f, // Bottom-right
 GLuint VAO;
 GLCall(glGenVertexArrays(1, &VAO));

 GLuint VBO;
 GLCall(glGenBuffers(1, &VBO));
 GLCall(glBindBuffer(GL_ARRAY_BUFFER, VBO));
 GLCall(glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), nullptr, GL_DYNAMIC_DRAW));

 GLCall(glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), (void*)0));

 float degree = 0.01f;

 bool quit = false;
 while (!quit) {

     glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(vertices), vertices);
     GLCall(glDrawArrays(GL_TRIANGLES, 0, 3));
     glm::mat4 rotationMatrix = glm::rotate(glm::mat4(1.0f), glm::radians(degree), glm::vec3(0.0f, 0.0f, 1.0f));
     for (int i = 0; i < 3; i += 2) {
         glm::vec4 vertex(vertices[i], vertices[i + 1], 0.0f, 1.0f);
         vertex = rotationMatrix * vertex;
         vertices[i] = vertex.x;
         vertices[i + 1] = vertex.y;

Shader vertex code:

#shader vertex
 #version 330 core

 layout(location = 0) in vec2 position;
 uniform mat4 u_ProjectionMatrix;

 void main()
     gl_Position = u_ProjectionMatrix * vec4(position.x, position.y, 0.0, 1.0); 

Btw. the u_ProjectionMatrix is keeping content on the same place, in case a resize window.

Function which handles window resize:

void changeViewport(int w, int h, int wStart, int wHeight, ShaderProgram& shaderProgram) {
    glViewport(0, 0, w, h);

    float wRatio = (float)w / wStart;
    float hRatio = (float)h / wHeight;

    // Pomocí nového poměru můžete vytvořit projekční matici pro zachování stejného zobrazení.
    glm::mat4 projection = glm::ortho(-wRatio, wRatio, -hRatio, hRatio, -1.0f, 1.0f);
    shaderProgram.SetUniformMatrix4fv("u_ProjectionMatrix", 1, GL_FALSE, glm::value_ptr(projection));

Thanks for the help guys.

I have found an unsuitable aspcect in your code. first you should send datas from CPU to GPU only once; second you should send projection new metrix again and again to change the position vertexs.