Please tell us what exactly you are doing step by step.
Also: how do you determine the size of the images?
That preview is pretty much meaningless. Export the images and check their size in an image editor such as Photoshop or an app such as Bridge.
I am creating some emails for my company and I need to export the assets
I created an artboard 600 pixels wide
Inside the drawing board I have an image with 480 (width) x 341 (height)
when I export this image, the dimensions show that it really has 480 x 341, but it is very small, look:
30 years ago 480 pixels was huge. nowadays it's small.
Have you looked in Image > Image size (in Photoshop)? Because your imression of this doesn't mater that much. Only thing that matters is the numbers.
When Photoshop displays an image at 100%, every pixel in the image is displayed by one pixel on screen.
You have a high resolution monitor that uses double the amount of pixels (144 ppi) so 72 ppi images are small in Photoshop.
Drag your image into a browser to see how it will be displayed on a web page.
If you want your images to look good on high res displays, export them at 2X the default resolution; 144 ppi (which is 2X in export for screens.