As you have pointed out. when you have only 6 anterior teeth Veneered you do tend to get the affect that we refer to in Dentistry as Collapsed Buccal Corridors. This can be due to the inclination of your existing premolars and molars, a colour discrepancy between the teeth treated up front and those in the buccal corridor or over-bulking of Veneers or crowns on the anterior 6 teeth.
From the pictures it appears like you have a wonderful smile but as you have noted the corridors are a bit dark in the photos. As far as comparing to the other pictures you have to understand that others may have a wider mouth in general so will have a wider smile. You can only work with the anatomy you have in terms of width of your mouth. But, as you have noted, including all the teeth in your smile (into the buccal corridors) can help the appearance of a wider smile.
When it comes to smile design there are many many factors that play into the overall appearance of your smile. Buccal corridors are of course an important part of the final look of your smile but attention also has to be paid to all the other variables that come into play. Inclinations, emergence profiles, gingival architecture, embrasure forms, line angles etc , just to name a few. A dentist, with proper training and experience working with a high quality dental lab will be able to optimize these parameters for you and provide you with what you are looking for, or something very very close.
As far as widening you smile outside of the Veneer work you are getting done, there is the possibility of opening up your smile with some Botox. This would require you working with a Botox practitioner who understands smile design and getting some treatment in your Depressor Anguli Oris muscles. If this is a route you decide to explore it is very important you work with someone who knows both smile design and botox or have your dentists and botox provider in close contact. done separately, the two procedures can work against each other as opposed to enhancing each other.