After I changed the slider image, why the image is it appearing smaller?

There is nothing to worry about, you can use the size (840x450 Px) slider for good image quality, and your image quality will remain good. 

Login Your wordpress Dashboard go appearance =>> Customizer =>> Frontpage Section =>> Slider Section =>> Slide, Upload jpg Image.

Image2

There is nothing to worry about, you can use the size (345x400 Px) slider for good image quality, and your image quality will remain good. 

Login Your wordpress Dashboard go appearance =>> Customizer =>> Frontpage Section =>> Slider Section =>> Slide, Upload Png Image.