When I say "blood cells", I mean "all blood cells", not specifically "red blood cells." Anywhere your blood plasma flows, all types of blood cells are carried along with it.

As such, to prevent infarction, every capillary in your body must be at least wide enough, in its narrowest state, to still accommodate the passage of the largest blood cell type the body produces, in its largest state. (Which, for us humans, is probably something like "a neutrophil that is bloated from just having consumed a large bacterium.")

The neutrophils and macrophages don't reliably know to exit the bloodstream when they're bloated?

Even if they did, they still might have ended up catching and eating the bacterium right at your fingertip. (Heck, that's not even an edge-case — fingertips and other extremeties served by the tiniest of bloodflow channels, get wounded and infected pretty often!)

Think of it like: what would civic street sizing regulations look like, if fire trucks — already the longest thing most residential streets need to accommodate — had to rapidly reconfigure and redeploy into an even longer shape, while sitting there on the street, to do their job; and then were stuck in this state until they made it back to the depot?

I see. Thanks.

I think derefr might be talking about the cells that form the walls of the capillaries being bigger, so you can't really fit them in the places you need them, and if you tried they'd be too narrow.

Except replace "you" with evolution and delete "tried".