SUMMARY

Screenshots made with spectacle when using a scale ratio are downscaled.

I believe the fix needs to be made in kwin/effects/screenshot/screenshot.cpp:607 :

QImage ScreenShotEffect::blitScreenshot(const QRect &geometry)
{
    QImage img;
    if (effects->isOpenGLCompositing())
    {
        img = QImage(geometry.size(), QImage::Format_ARGB32);
        if (GLRenderTarget::blitSupported() && !GLPlatform::instance()->isGLES()) {
            GLTexture tex(GL_RGBA8, geometry.width(), geometry.height());
            GLRenderTarget target(tex);
            target.blitFromFramebuffer(geometry);
            // copy content from framebuffer into image
            tex.bind();
            glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid*)img.bits());
            tex.unbind();
        } else {
            glReadPixels(0, 0, img.width(), img.height(), GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid*)img.bits());
        }
        ScreenShotEffect::convertFromGLImage(img, geometry.width(), geometry.height());
    }

To have an output image whose output image is scaled up.

STEPS TO REPRODUCE
1. With a scaling ratio of 2 for instance (physical res is 3840x2160)
2. Take a fullscreen screenshot with spectacle 

OBSERVED RESULT
Screenshot size is 1920*1080.

EXPECTED RESULT
Screenshot size is 3840x2160.

SOFTWARE/OS VERSIONS
Operating System: Kubuntu 19.10
KDE Plasma Version: 5.17.80
KDE Frameworks Version: 5.64.0
Qt Version: 5.12.4
Kernel Version: 5.3.0-23-generic