Seattle has:

Medium sized international commercial sea port.

Boeing Field, half of which is a military only zone.

Largest manufactorium of aircraft in the world a bit north in Everett, more stuff in Kent.

Also in Everett is a deep water west coast Naval base.

Nearby to Seattle is JBLM, huge military base.

Nearby is also Fairchild and Kitsap major Naval bases. Fairchild has Navy's ELINT aircraft, Kitsap has a whole lotta submarines.

Major general convergence point for road traffic, blow apart I5, imagine 405 traffic after that.

Pretty major concentration of transformers for electrical for the whole region.

Major industrial railyard, medium sized passenger one.

... And a whole lot of major internet undersea tunnels connect to the rest of the world, and the rest of the US, in a building in downtown Seattle, which of course also has a couple floors dedicated to every 3 letter agency you've ever heard of.

It completely makes sense for an adversary to nuke the Everett to Seattle to Tacoma area out of existence.

Could be wrong, but I believe Seattle has such a large looking symbol because it doesn't have a bunch of other ones around it. I think the size of the symbol is the same as the size of the rest of the civilian targets. Jacksonville also stood out to me at first but I think New York's is the same size.

Edit: there's two sizes for the symbols, large and small. Seattle's is the same size as Portland's.