Q: Sometimes my post with image seems to be very small and centered to the middle of the page. Why is that?

Displays resolution and resolution of an image are the determining factors. 

An image is not stretched to fit the screen size because that would cause the image to blur. Valotalive reads resolution of an image and fits it to meet the screen resolution.

In other words if you have 4K display you'd need to have image with bigger resolution than with Full HD display. 

Workaround: Insert your image to a PowerPoint or a Slides presentation, save the presentation as PDF and post the PDF to the displays using My Content. This way your image will fit the display the same way as it fits the presentation.