Display advertising in the United States runs on a small set of repeated dimensions rather than on whatever shape a designer prefers. That standardization is a trading convenience before it is a design decision.
Standard sizes exist so inventory can be pooled
A campaign that buys across the open web touches thousands of sites built by unrelated publishers. Each of those sites reserved space in its page template long before any advertiser arrived.
If every advertiser supplied a different shape, no publisher could guarantee a fit. Agreeing on a shared set of dimensions turns scattered page slots into a single pool of interchangeable inventory.
Trade bodies in the American market published those shared specifications, and ad servers, exchanges and content management systems all built support for the same list.
Production cost falls sharply with fewer shapes
Creative is produced per size, not per placement. An advertiser supporting six dimensions builds six versions of every message, then rebuilds all six whenever the offer or the legal line changes.
Because that cost repeats through the life of a campaign, teams concentrate on the handful of sizes that carry most impressions and skip the long tail entirely.
The result is circular and self-reinforcing. Publishers keep the slots that advertisers fill, advertisers build for the slots publishers keep, and unusual dimensions stay unsold.
Responsive units loosened the rule without replacing it
Platforms later introduced formats where the advertiser uploads images, headlines and logos separately and the system assembles a unit sized to the available space.
That removes the production burden for many buyers, but it does not remove the underlying constraint. The assembled unit still has to land inside a slot the publisher reserved in advance.
Responsive formats therefore widened the range of shapes that can be filled rather than freeing creative from shape altogether.
Size changes what a placement is worth
Larger units and units that sit within the article body generally command higher prices, because they are more likely to be seen and harder to scroll past unnoticed.
Buyers who evaluate a placement only on cost per thousand impressions often find that the cheapest sizes are cheap for a measurable reason.
Comparing performance within a single size is usually more informative than comparing across sizes, since dimension and position influence outcomes before creative does.
Mobile fragmented the list again
Phone screens made some traditional desktop dimensions unusable and introduced new ones suited to a narrow vertical column and to scrolling rather than clicking.
Apps added a further layer, since in-app inventory is sold through software development kits with their own supported formats and their own rendering behavior.
Advertisers now maintain parallel size sets for desktop web, mobile web and apps, which is why a single display campaign involves more creative files than its simplicity suggests.