uploaded slider/post thumbnails: why cropped/resized?

when I upload an image to slider or as post thumbnails, it does’nt use my original -optimized and dimensioned – one, but it upscale and crops it.

example:

I upload this image to i-craft slider
https://www.dynamicsystem.fe.it/wp-content/uploads/2016/05/dynamic-system-montaggio-incluso.jpg

then I have this one, oversampled and used.

https://www.dynamicsystem.fe.it/wp-content/uploads/2016/05/dynamic-system-montaggio-incluso-2400×840.jpg

same on post thumbnails. example:

I upload this

https://www.dynamicsystem.fe.it/wp-content/uploads/2016/06/iva-ico.jpg

and the frontpage uses this

https://www.dynamicsystem.fe.it/wp-content/uploads/2016/06/iva-ico-600×600.jpg

there is a specific reason? can this system be disabled?